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Wikipedia

Earth

January 01, 1970

"Planet Earth" redirects here. For other uses, see Earth (disambiguation) and Planet Earth (disambiguation).

Earth is the third planet from the Sun and the only astronomical object known to harbor life. This is enabled by Earth being a water world, the only one in the Solar System sustaining liquid surface water. Almost all of Earth's water is contained in its global ocean, covering 70.8% of Earth's crust. The remaining 29.2% of Earth's crust is land, most of which is located in the form of continental landmasses within Earth's land hemisphere. Most of Earth's land is somewhat humid and covered by vegetation, while large sheets of ice at Earth's polar deserts retain more water than Earth's groundwater, lakes, rivers and atmospheric water combined. Earth's crust consists of slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth has a liquid outer core that generates a magnetosphere capable of deflecting most of the destructive solar winds and cosmic radiation.

Earth
The Blue Marble, Apollo 17, December 1972
Designations
The world, the globe, Sol III, Terra, Tellus, Gaia, Mother Earth
AdjectivesEarthly, terrestrial, terran, tellurian
Symbol and
Orbital characteristics
Epoch J2000[n 1]
Aphelion152097597 km (94509065 mi)
Perihelion147098450 km (91402740 mi)[n 2]
149598023 km (92955902 mi)[1]
Eccentricity0.0167086[1]
365.256363004 d[2]
(1.00001742096 aj)
29.7827 km/s[3]
(107218 km/h; 66622 mph)
358.617°
Inclination
−11.26064° – J2000 ecliptic[3]
2023-Jan-04[5]
114.20783°[3]
Satellites1, the Moon
Physical characteristics
Mean radius
6371.0 km (3958.8 mi)[6]
Equatorial radius
6378.137 km (3963.191 mi)[7][8]
Polar radius
6356.752 km (3949.903 mi)[9]
Flattening1/298.257222101 (ETRS89)[10]
Circumference
  • 510072000 km2
    (196940000 sq mi)[12][n 4]
  • Land: 148940000 km2
    (57510000 sq mi)
  • Water: 361132000 km2
    (139434000 sq mi)
Volume1.08321×1012 km3 (2.59876×1011 cu mi)[3]
Mass5.972168×1024 kg (1.31668×1025 lb)[13]
Mean density
5513 kg/m3
(0.1992 lb/cu in)[3]
9.80665 m/s2
(32.1740 ft/s2)[14]
0.3307[15]
11.186 km/s (40270 km/h; 25020 mph)[3]
1.0 d
(24h 00 m 00s)
0.99726968 d[16]
(23h 56 m 4.100s)
Equatorial rotation velocity
0.4651 km/s[17]
(1674.4 km/h; 1040.4 mph)
23.4392811°[2]
Albedo
Temperature255 K (−18 °C; −1 °F)
(blackbody temperature)[18]
Surface temp. 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
Surface equivalent dose rate0.274 μSv/h[22]
−3.99
Atmosphere
Surface pressure
101.325 kPa (at sea level)
Composition by volume
Source:[3]

Earth has a dynamic atmosphere, which sustains Earth's surface conditions and protects it from most meteoroids and UV-light at entry. It has a composition of primarily nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly carbon dioxide (CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy 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 atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with the equatorial region receiving more sunlight than the polar regions) drive atmospheric and ocean currents, producing a global climate system with different climate regions, and a range of weather phenomena such as precipitation, allowing components such as nitrogen to cycle.

Earth is rounded into 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. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). 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 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, causes tides and gradually slows Earth's rotation. Tidal locking has made the Moon always face Earth with the same side.

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 has been altering Earth's atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly impacted the planet's environment. Humanity's current impact on Earth's climate and biosphere is unsustainable, threatening the livelihood of humans and many other forms of life, and causing widespread extinctions.[23]

Etymology

The Modern English word Earth developed, via Middle English, from an Old English noun most often spelled eorðe.[24] It has cognates in every Germanic language, and their ancestral root has been reconstructed as *erþō. In its earliest attestation, the word eorðe was used to translate the many senses of Latin terra and Greek γῆ gē: 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.[25]

Historically, "Earth" has been written in lowercase. Beginning with the use of 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 earth" remain common.[24] House styles now vary: Oxford spelling recognizes 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 name Terra /ˈtɛrə/ occasionally is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others,[27] while in poetry Tellus /ˈtɛləs/ has been used to denote personification of the Earth.[28] 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: /ˈdʒiː.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 /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 the Latin Terra comes terran /ˈtɛrən/,[30] terrestrial /təˈrɛstriəl/,[31] and (via French) terrene /təˈriːn/,[32] and from the Latin Tellus comes tellurian /tɛˈlʊəriən/[33] and telluric.[34]

Natural history

Main articles: History of Earth and Timeline of natural history

Formation

Further information: Early Earth and Hadean
 
A 2012 artistic impression of the early Solar System's protoplanetary disk from which Earth and other Solar System bodies were formed

The oldest material found in the Solar System is dated to 4.5682+0.0002
−0.0004 Ga (billion years) ago.[35] By 4.54±0.04 Ga the primordial Earth had formed.[36] 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.[37]

Estimates of the age of the Moon range from 4.5 Ga to significantly younger.[38] 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.[39] It hit Earth with a glancing blow and some of its mass merged with Earth.[40][41] 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.[42]

After formation

Main article: Geological history of Earth

Earth's atmosphere and oceans were formed by volcanic activity and outgassing.[43] Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets.[44] Sufficient water to fill the oceans may have been on Earth since it formed.[45] In this model, atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity.[46] By 3.5 Ga, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[47]

 
Pale orange dot, an artist's impression of Early Earth, featuring its tinted orange methane-rich early atmosphere[48]

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.[49] 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.[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 the Archean, forming the bulk of the continental crust that now exists,[53][54] 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.[55]

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.[56]

The most recent pattern of ice ages began about 40 Ma,[57] and then intensified during the Pleistocene about 3 Ma.[58] 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.[59] 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.[60]

Origin of life and evolution

Main articles: Origin of life, Earliest known life forms, and History of life

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.[61] 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.[62] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[63] 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.[64] Among the earliest fossil evidence for life is microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia,[65] biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland,[66] and remains of biotic material found in 4.1 billion-year-old rocks in Western Australia.[67][68] The earliest direct evidence of life on Earth is contained in 3.45 billion-year-old Australian rocks showing fossils of microorganisms.[69][70]

 
An 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 half as far as it is today, appearing 2.8 times larger and producing strong tides.[71]

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.[72][73] Following the Cambrian explosion, 535 Ma, there have been at least five major mass extinctions and many minor ones.[74] Apart from the proposed current Holocene extinction event, the most recent was 66 Ma, when an asteroid impact triggered 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 past 66 Mys, and several million years ago, an African ape species 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 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.[77]

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 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%.[78] 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.[79][80] The lack of vegetation will result in the loss of oxygen in the atmosphere, making 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 a runaway 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 the mantle, due to reduced steam venting from mid-ocean ridges.[82][83]

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.[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, and Geomorphology
See also: List of highest mountains on Earth
 
Earth's western hemisphere showing topography relative to Earth's center instead of to mean sea level, as in common topographic maps

Earth has a rounded shape, through hydrostatic equilibrium,[85] with an average diameter of 12,742 kilometers (7,918 mi), making it the fifth largest planetary sized and largest terrestrial object of the Solar System.[86]

Due to Earth's rotation it has the shape of an ellipsoid, bulging at its Equator; its diameter is 43 kilometers (27 mi) longer there than at its poles.[87][88] Earth's shape furthermore has local topographic variations. Though the largest local 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] Since Earth's surface is farthest out from Earth's center of mass at its equatorial bulge, the summit of the volcano Chimborazo in Ecuador (6,384.4 km or 3,967.1 mi) is its farthest point out.[92][93] Parallel to the rigid land topography the Ocean exhibits a more dynamic topography.[94]

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.[95]

Surface

Further information: Planetary surface, Land cover, Land, Pedosphere, Ocean, Sea, Cryosphere, and Peplosphere
 
A composite image 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 of Antarctic sea ice (grey) covering the Antarctic or Southern Ocean and the Antarctic ice sheet (white) covering Antarctica.
 
Relief of Earth'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 two hemispheres: by latitude into the polar Northern and Southern hemispheres; or by longitude into the continental Eastern and Western hemispheres.[citation needed]

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 the world ocean,[97][98] and makes Earth with its dynamic hydrosphere a water world[99][100] or ocean 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 covers Earth's oceanic crust, with the shelf seas covering the shelves of the continental crust to a lesser extent. The oceanic crust forms large oceanic basins with features like abyssal plains, seamounts, submarine volcanoes,[87] oceanic trenches, submarine canyons, oceanic plateaus, and a globe-spanning mid-ocean ridge system.[citation needed]

At Earth's polar regions, the ocean surface is covered by seasonally variable amounts of sea ice that often connects with polar land, permafrost and ice sheets, forming polar ice caps.[citation needed]

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 continental landmasses, which are (in descending order): Africa-Eurasia, America (landmass), Antarctica, and Australia (landmass).[104][105][106] These landmasses are further broken down and grouped into the continents. The terrain of the land surface varies greatly and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from a 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).[107]

Land can be covered by surface water, snow, ice, artificial structures or vegetation. Most of Earth's land hosts vegetation,[108] but considerable amounts of land are ice sheets (10%,[109] not including the equally large area of land under permafrost)[110] or deserts (33%)[111]

The pedosphere is the outermost layer of Earth's land surface and is composed of soil and subject to soil formation processes. Soil is crucial for land to be arable. Earth's total arable land is 10.7% of the land surface, with 1.3% being permanent cropland.[112][113] 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.[114]

The land surface and the ocean floor form the top of Earth's crust, which together with parts of the upper mantle form Earth's lithosphere. Earth's crust may be divided into oceanic and continental crust. Beneath the ocean-floor sediments, the oceanic crust is predominantly basaltic, while the continental crust may include lower density materials such as granite, sediments and metamorphic rocks.[115] Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the mass of the crust.[116]

Earth's surface topography comprises both the topography of the ocean surface, and the shape of Earth's land surface. The submarine terrain of the ocean floor has an average bathymetric depth of 4 km, and is as varied as the terrain above sea level. Earth's surface is continually being shaped by internal plate tectonic processes including earthquakes and volcanism; by weathering and erosion driven by ice, water, wind and temperature; and by biological processes including the growth and decomposition of biomass into soil.[117][118]

Tectonic plates

Main article: Plate tectonics
 
Earth's major plates, which are:[119]

Earth's mechanically rigid outer layer of Earth's crust and upper mantle, 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.[120] 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.[121]

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.[122][123] By comparison, the oldest dated continental crust is 4,030 Ma,[124] 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.[50]

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)[125] 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).[126]

Internal structure

Main article: Internal structure of Earth
Geologic layers of Earth[127]
 
Illustration of Earth's cutaway, not to scale
Depth[128]
(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.[129] 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.[130]

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.[131] 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.[132] 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.[citation needed]

Among the Solar System's planetary-sized objects Earth is the object with the highest density.[citation needed]

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% by mass), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to gravitational 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.[133][49] The most common rock constituents of the crust are oxides. Over 99% of the crust is composed of various oxides of eleven elements, principally oxides containing silicon (the silicate minerals), aluminium, iron, calcium, magnesium, potassium, or sodium.[134][133]

Internal heat

Main article: Earth's internal heat budget
 
A map of heat flow from Earth's interior to the surface of Earth's crust, mostly along the oceanic ridges

The major heat-producing isotopes within Earth are potassium-40, uranium-238, and thorium-232.[135] At the center, the temperature may be up to 6,000 °C (10,830 °F),[136] and the pressure could reach 360 GPa (52 million psi).[137] 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.[138][139]

The mean heat loss from Earth is 87 mW m−2, for a global heat loss of 4.42×1013 W.[140] 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.[141] 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.[142]

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.[143]

Magnetic field

Main article: Earth's magnetic field
 
A schematic view of Earth's magnetosphere with solar wind flowing 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).[144] 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.[145][146]

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.[147] 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.[148] Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.[149][150] 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,[151] and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.[152][153]

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.[154]

Orbit and rotation

Rotation

Main article: Earth's rotation
 
Satellite time lapse imagery of Earth's rotation 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).[155] 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.[156][157]

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.[2][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).[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[158]

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.[159][160]

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, the 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.[citation needed]

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.[citation needed]

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.[3]

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.[3][161]

The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.[162][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.[162] 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.[163]

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 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.[citation needed]

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.[164] 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.[165][166]

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.[167]

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.[168] 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.[169]

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.[170][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.[171]

Earth–Moon system

Further information: Satellite system (astronomy)

Moon

Main articles: Moon, Lunar theory, and Orbit of the Moon
 
Earth and the Moon as seen from Mars by the Mars Reconnaissance Orbiter
 
View of Earth from the Moon by the Lunar 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, although Charon is larger relative to the dwarf planet Pluto.[172][173] The natural satellites of other planets are also referred to as "moons", after Earth's.[174] 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 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]

The gravitational attraction between Earth and the Moon causes tides on Earth.[175] 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.[176] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases.[177] 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.[178] 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.[179]

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.[180] 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.[181][182]

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.[160] This allows total and annular solar eclipses to occur on Earth.[183]

On 1 November 2023, scientists reported that, according to computer simulations, remnants of a protoplanet, named Theia, could be inside the Earth, left over from a collision with the Earth in ancient times, and afterwards becoming the Moon.[184][185]

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.[186][187] A trojan asteroid companion, 2010 TK7, is librating around the leading Lagrange triangular point, L4, in Earth's orbit around the Sun.[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.[190] There are also inoperative satellites, including Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked space debris.[n 13] Earth's largest artificial satellite is the International Space Station.[191]

Hydrosphere

Main 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, 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 down to a depth of 2,000 m (6,600 ft).[citation needed]

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).[192] 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).[193] About 97.5% of the water is saline; the remaining 2.5% is fresh water.[194][195] Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.[196] The remaining 30% is ground water, 1% surface water (covering only 2.8% of Earth's land)[197] and other small forms of fresh water deposits such as permafrost, water vapor in the atmosphere, biological binding, etc.[198][199]

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.[200] 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.[201]

The average salinity of Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt).[202] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[203] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[204] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[205] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.[206]

The abundance of water, particularly liquid 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.[207] 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.[208]

Atmosphere

Main article: Atmosphere of Earth
 
A view of Earth with different layers of its atmosphere visible: the troposphere with its clouds casting shadows, a band of stratospheric blue sky at the horizon, and a line of green airglow of the lower thermosphere 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),[209] with a scale height of 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.[209] Water vapor content varies between 0.01% and 4%[209] but averages about 1%.[3] Clouds cover around two-thirds of Earth's surface, more so over oceans than land.[210] 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.[211]

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today.[62] 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.[212] 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.[213] 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),[214] and life on Earth probably would not exist in its current form.[215]

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 Köppen climate classifications

Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.[216] 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.[217] 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.[218]

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°.[219] 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.[220]

Earth receives 1361 W/m2 of solar irradiance.[221][222] 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.[223] 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.[224]

Further factors that affect a location's climates are its proximity to oceans, the oceanic and atmospheric circulation, and topology.[225] 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.[226] 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.[227] Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas.[228]

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.[218] 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.[229]

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.[219] The Köppen system rates regions based on observed temperature and precipitation.[230] 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.[231][232]

Upper atmosphere

 
Earth's night-side upper atmosphere appearing from the bottom as bands of afterglow illuminating the troposphere in orange with silhouettes of clouds, and the stratosphere in white and blue. Next the mesosphere (pink area) extends to the orange and faintly green line of the lowest airglow, at about one hundred kilometers at the edge of space and the lower edge of the thermosphere (invisible). Continuing with green and red bands of aurorae streching over several hundred kilometers.

The upper atmosphere, the atmosphere above the troposphere,[233] is usually divided into the stratosphere, mesosphere, and thermosphere.[213] 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.[234] 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.[235]

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.[236] 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.[237] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[238] 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.[239]

Life on Earth

Main articles: Biosphere and History of life
 
An animation of the changing density of productive vegetation 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 has ever been habitable for life. Earth's life developed in Earth's early bodies of water some hundred million years after Earth formed.[citation needed]

Earth's life has been shaping and inhabiting many particular ecosystems on Earth and has eventually expanded globally forming an overarching biosphere.[240] Therefore, life has impacted Earth, significantly altering Earth's atmosphere and surface over long periods of time, causing changes like the Great Oxidation Event.[241]

Earth's life has over time greatly diversified, allowing the biosphere to have different biomes, which are inhabited by comparatively similar plants and animals.[242] The different biomes developed 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.[243]

Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain a metabolism.[244] Plants and other organisms take up nutrients from water, soils and the atmosphere. These nutrients are constantly recycled between different species.[245]

 
A High Desert storm, sweeps across the Mojave

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.[246] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, blizzards, floods, droughts, wildfires, and other calamities and disasters.[247] 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.[248] Human activities release greenhouse gases into the atmosphere which cause global warming.[249] 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.[250]

Human geography

Main 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.[251] In the 20th century Antarctica had been the last continent to see a first and until today limited human presence.[citation needed]

Human population has since the 19th century grown exponentially to seven billion in the early 2010s,[252] and is projected to peak at around ten billion in the second half of the 21st century.[253] Most of the growth is expected to take place in sub-Saharan Africa.[253]

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,[254] 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.[255] Furthermore, since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century.[256]

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. Since the second half of the 20th century, some hundreds of humans have temporarily stayed beyond Earth, a tiny fraction of whom have reached another celestial body, the Moon.[257][258]

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 by sovereign states (countries) separated by political borders, and 205 such states exist today,[259] with only parts of Antarctica and a few small regions remaining unclaimed.[260] Most of these states together form the United Nations, the leading worldwide intergovernmental organization,[261] which extends human governance over the ocean and Antarctica, and therefore all of Earth.[citation needed]

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.[262] Those termed non-renewable resources, such as fossil fuels, are only replenished over geological timescales.[263] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[264] These deposits are used by humans both for energy production and as feedstock for chemical production.[265] 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.[266] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[267]

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.[268] 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.[269] Of the 12–14% of ice-free land that is used for croplands, 2 percentage points were irrigated in 2015.[270] Humans use building materials to construct shelters.[271]

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.[272]

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.[249][273] It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.[274] 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.[250]

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.[275][276] 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.[277]

Cultural and historical viewpoint

Main articles: Earth in culture and Earth in science fiction
 
Tracy Caldwell Dyson, a NASA astronaut, observing Earth from the Cupola module at the International Space Station on 11 September 2010

Human cultures have developed many views of the planet.[278] The standard astronomical symbols of Earth are a quartered circle,  ,[279] 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.[280] Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.[280] 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.[281][282][283]

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.[284][285] In particular, this caused a realization of the scope of effects from human activity on Earth's environment. Enabled by science, particularly Earth observation,[286] humans have started to take action on environmental issues globally,[287] acknowledging the impact of humans and the interconnectedness of Earth's environments.[citation needed]

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.[288][289] 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.[290]

It was only during the 19th century that geologists realized Earth's age was at least many millions of years.[291] 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.[292][293]

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. ^ 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. ^ 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.
  4. ^ 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.
  5. ^ Source for minimum,[19] mean,[20] and maximum[21] surface temperature
  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): 359–361. 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.
  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. Retrieved 18 April 2018.

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January 01, 1970
earth, planet, redirects, here, other, uses, disambiguation, planet, disambiguation, third, planet, from, only, astronomical, object, known, harbor, life, this, enabled, being, water, world, only, solar, system, sustaining, liquid, surface, water, almost, wate. Planet Earth redirects here For other uses see Earth disambiguation and Planet Earth disambiguation Earth is the third planet from the Sun and the only astronomical object known to harbor life This is enabled by Earth being a water world the only one in the Solar System sustaining liquid surface water Almost all of Earth s water is contained in its global ocean covering 70 8 of Earth s crust The remaining 29 2 of Earth s crust is land most of which is located in the form of continental landmasses within Earth s land hemisphere Most of Earth s land is somewhat humid and covered by vegetation while large sheets of ice at Earth s polar deserts retain more water than Earth s groundwater lakes rivers and atmospheric water combined Earth s crust consists of slowly moving tectonic plates which interact to produce mountain ranges volcanoes and earthquakes Earth has a liquid outer core that generates a magnetosphere capable of deflecting most of the destructive solar winds and cosmic radiation EarthThe Blue Marble Apollo 17 December 1972DesignationsAlternative namesThe world the globe Sol III Terra Tellus Gaia Mother EarthAdjectivesEarthly terrestrial terran tellurianSymbolandOrbital characteristicsEpoch J2000 n 1 Aphelion152097 597 km 94509 065 mi Perihelion147098 450 km 91402 740 mi n 2 Semi major axis149598 023 km 92955 902 mi 1 Eccentricity0 0167086 1 Orbital period sidereal 365 256363 004 d 2 1 000017 420 96 aj Average orbital speed29 7827 km s 3 107218 km h 66622 mph Mean anomaly358 617 Inclination7 155 Sun s equator 1 57869 invariable plane 4 0 00005 J2000 eclipticLongitude of ascending node 11 26064 J2000 ecliptic 3 Time of perihelion2023 Jan 04 5 Argument of perihelion114 20783 3 Satellites1 the MoonPhysical characteristicsMean radius6371 0 km 3958 8 mi 6 Equatorial radius6378 137 km 3963 191 mi 7 8 Polar radius6356 752 km 3949 903 mi 9 Flattening1 298 257222 101 ETRS89 10 Circumference40075 017 km 24901 461 mi equatorial 8 40007 86 km 24859 73 mi meridional 11 n 3 Surface area510072 000 km2 196940 000 sq mi 12 n 4 Land 148940 000 km2 57510 000 sq mi Water 361132 000 km2 139434 000 sq mi Volume1 08321 1012 km3 2 59876 1011 cu mi 3 Mass5 972168 1024 kg 1 31668 1025 lb 13 Mean density5513 kg m3 0 1992 lb cu in 3 Surface gravity9 80665 m s2 32 1740 ft s2 14 Moment of inertia factor0 3307 15 Escape velocity11 186 km s 40270 km h 25020 mph 3 Synodic rotation period1 0 d 24h 00 m 00s Sidereal rotation period0 997269 68 d 16 23h 56 m 4 100s Equatorial rotation velocity0 4651 km s 17 1674 4 km h 1040 4 mph Axial tilt23 4392811 2 Albedo0 367 geometric 3 0 306 Bond 3 Temperature255 K 18 C 1 F blackbody temperature 18 Surface temp min mean maxCelsius n 5 89 2 C 14 76 C 56 7 CFahrenheit 128 5 F 58 568 F 134 0 FSurface equivalent dose rate0 274 mSv h 22 Absolute magnitude H 3 99AtmosphereSurface pressure101 325 kPa at sea level Composition by volume78 08 nitrogen dry air 20 95 oxygen dry air 1 water vapor variable 0 9340 argon0 0415 carbon dioxide0 00182 neon0 00052 helium0 00017 methane0 00011 krypton0 00006 hydrogen Source 3 Earth has a dynamic atmosphere which sustains Earth s surface conditions and protects it from most meteoroids and UV light at entry It has a composition of primarily nitrogen and oxygen Water vapor is widely present in the atmosphere forming clouds that cover most of the planet The water vapor acts as a greenhouse gas and together with other greenhouse gases in the atmosphere particularly carbon dioxide CO2 creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy 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 atmospheric pressure Differences in the amount of captured energy between geographic regions as with the equatorial region receiving more sunlight than the polar regions drive atmospheric and ocean currents producing a global climate system with different climate regions and a range of weather phenomena such as precipitation allowing components such as nitrogen to cycle Earth is rounded into 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 Earth rotates around its own axis in slightly less than a day in about 23 hours and 56 minutes 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 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 causes tides and gradually slows Earth s rotation Tidal locking has made the Moon always face Earth with the same side 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 has been altering Earth s atmosphere and surface leading to the Great Oxidation Event two billion years ago Humans emerged 300 000 years ago in Africa and have spread across every continent on Earth Humans depend on Earth s biosphere and natural resources for their survival but have increasingly impacted the planet s environment Humanity s current impact on Earth s climate and biosphere is unsustainable threatening the livelihood of humans and many other forms of life and causing widespread extinctions 23 Contents 1 Etymology 2 Natural history 2 1 Formation 2 2 After formation 2 3 Origin of life and evolution 2 4 Future 3 Physical characteristics 3 1 Size and shape 3 2 Surface 3 3 Tectonic plates 3 4 Internal structure 3 5 Chemical composition 3 6 Internal 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 24 It has cognates in every Germanic language and their ancestral root has been reconstructed as erthō In its earliest attestation the word eorde was 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 25 Historically Earth has been written in lowercase Beginning with the use of 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 earth remain common 24 House styles now vary Oxford spelling recognizes 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 name Terra ˈ t ɛr e occasionally is used in scientific writing and especially in science fiction to distinguish humanity s inhabited planet from others 27 while in poetry Tellus ˈ t ɛ l e s has been used to denote personification of the Earth 28 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 29 There are a number of adjectives for the planet Earth The word earthly is derived from Earth From the Latin Terra comes terran ˈ t ɛr e n 30 terrestrial t e ˈ r ɛ s t r i e l 31 and via French terrene t e ˈ r iː n 32 and from the Latin Tellus comes tellurian t ɛ ˈ l ʊer i e n 33 and telluric 34 Natural historyMain articles History of Earth and Timeline of natural history Formation Further information Early Earth and Hadean nbsp A 2012 artistic impression of the early Solar System s protoplanetary disk from which Earth and other Solar System bodies were formed The oldest material found in the Solar System is dated to 4 5682 0 0002 0 0004 Ga billion years ago 35 By 4 54 0 04 Ga the primordial Earth had formed 36 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 37 Estimates of the age of the Moon range from 4 5 Ga to significantly younger 38 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 39 It hit Earth with a glancing blow and some of its mass merged with Earth 40 41 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 42 After formation Main article Geological history of Earth Earth s atmosphere and oceans were formed by volcanic activity and outgassing 43 Water vapor from these sources condensed into the oceans augmented by water and ice from asteroids protoplanets and comets 44 Sufficient water to fill the oceans may have been on Earth since it formed 45 In this model atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70 of its current luminosity 46 By 3 5 Ga Earth s magnetic field was established which helped prevent the atmosphere from being stripped away by the solar wind 47 nbsp Pale orange dot an artist s impression of Early Earth featuring its tinted orange methane rich early atmosphere 48 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 49 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 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 the Archean forming the bulk of the continental crust that now exists 53 54 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 55 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 56 The most recent pattern of ice ages began about 40 Ma 57 and then intensified during the Pleistocene about 3 Ma 58 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 59 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 60 Origin of life and evolution Main articles Origin of life Earliest known life forms and History of life 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 61 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 62 The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes 63 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 64 Among the earliest fossil evidence for life is microbial mat fossils found in 3 48 billion year old sandstone in Western Australia 65 biogenic graphite found in 3 7 billion year old metasedimentary rocks in Western Greenland 66 and remains of biotic material found in 4 1 billion year old rocks in Western Australia 67 68 The earliest direct evidence of life on Earth is contained in 3 45 billion year old Australian rocks showing fossils of microorganisms 69 70 nbsp An 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 half as far as it is today appearing 2 8 times larger and producing strong tides 71 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 72 73 Following the Cambrian explosion 535 Ma there have been at least five major mass extinctions and many minor ones 74 Apart from the proposed current Holocene extinction event the most recent was 66 Ma when an asteroid impact triggered 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 past 66 Mys and several million years ago an African ape species 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 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 77 Future Main article Future of Earth See also Global catastrophic risk nbsp 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 78 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 79 80 The lack of vegetation will result in the loss of oxygen in the atmosphere making 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 a runaway 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 the mantle due to reduced steam venting from mid ocean ridges 82 83 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 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 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 nbsp Earth s western hemisphere showing topography relative to Earth s center instead of to mean sea level as in common topographic maps Earth has a rounded shape through hydrostatic equilibrium 85 with an average diameter of 12 742 kilometers 7 918 mi making it the fifth largest planetary sized and largest terrestrial object of the Solar System 86 Due to Earth s rotation it has the shape of an ellipsoid bulging at its Equator its diameter is 43 kilometers 27 mi longer there than at its poles 87 88 Earth s shape furthermore has local topographic variations Though the largest local 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 Since Earth s surface is farthest out from Earth s center of mass at its equatorial bulge the summit of the volcano Chimborazo in Ecuador 6 384 4 km or 3 967 1 mi is its farthest point out 92 93 Parallel to the rigid land topography the Ocean exhibits a more dynamic topography 94 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 95 Surface Further information Planetary surface Land cover Land Pedosphere Ocean Sea Cryosphere and Peplosphere nbsp A composite image 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 of Antarctic sea ice grey covering the Antarctic or Southern Ocean and the Antarctic ice sheet white covering Antarctica nbsp Relief of Earth 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 two hemispheres by latitude into the polar Northern and Southern hemispheres or by longitude into the continental Eastern and Western hemispheres citation needed 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 the world ocean 97 98 and makes Earth with its dynamic hydrosphere a water world 99 100 or ocean 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 covers Earth s oceanic crust with the shelf seas covering the shelves of the continental crust to a lesser extent The oceanic crust forms large oceanic basins with features like abyssal plains seamounts submarine volcanoes 87 oceanic trenches submarine canyons oceanic plateaus and a globe spanning mid ocean ridge system citation needed At Earth s polar regions the ocean surface is covered by seasonally variable amounts of sea ice that often connects with polar land permafrost and ice sheets forming polar ice caps citation needed 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 continental landmasses which are in descending order Africa Eurasia America landmass Antarctica and Australia landmass 104 105 106 These landmasses are further broken down and grouped into the continents The terrain of the land surface varies greatly and consists of mountains deserts plains plateaus and other landforms The elevation of the land surface varies from a 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 107 Land can be covered by surface water snow ice artificial structures or vegetation Most of Earth s land hosts vegetation 108 but considerable amounts of land are ice sheets 10 109 not including the equally large area of land under permafrost 110 or deserts 33 111 The pedosphere is the outermost layer of Earth s land surface and is composed of soil and subject to soil formation processes Soil is crucial for land to be arable Earth s total arable land is 10 7 of the land surface with 1 3 being permanent cropland 112 113 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 114 The land surface and the ocean floor form the top of Earth s crust which together with parts of the upper mantle form Earth s lithosphere Earth s crust may be divided into oceanic and continental crust Beneath the ocean floor sediments the oceanic crust is predominantly basaltic while the continental crust may include lower density materials such as granite sediments and metamorphic rocks 115 Nearly 75 of the continental surfaces are covered by sedimentary rocks although they form about 5 of the mass of the crust 116 Earth s surface topography comprises both the topography of the ocean surface and the shape of Earth s land surface The submarine terrain of the ocean floor has an average bathymetric depth of 4 km and is as varied as the terrain above sea level Earth s surface is continually being shaped by internal plate tectonic processes including earthquakes and volcanism by weathering and erosion driven by ice water wind and temperature and by biological processes including the growth and decomposition of biomass into soil 117 118 Tectonic plates Main article Plate tectonics nbsp Earth s major plates which are 119 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 of Earth s crust and upper mantle 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 120 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 121 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 122 123 By comparison the oldest dated continental crust is 4 030 Ma 124 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 50 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 125 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 126 Internal structure Main article Internal structure of Earth Geologic layers of Earth 127 nbsp Illustration of Earth s cutaway not to scale Depth 128 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 Mohorovicic discontinuity 129 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 130 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 131 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 132 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 citation needed Among the Solar System s planetary sized objects Earth is the object with the highest density citation needed 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 by mass oxygen 30 1 silicon 15 1 magnesium 13 9 sulfur 2 9 nickel 1 8 calcium 1 5 and aluminium 1 4 with the remaining 1 2 consisting of trace amounts of other elements Due to gravitational 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 133 49 The most common rock constituents of the crust are oxides Over 99 of the crust is composed of various oxides of eleven elements principally oxides containing silicon the silicate minerals aluminium iron calcium magnesium potassium or sodium 134 133 Internal heat Main article Earth s internal heat budget nbsp A map of heat flow from Earth s interior to the surface of Earth s crust mostly along the oceanic ridges The major heat producing isotopes within Earth are potassium 40 uranium 238 and thorium 232 135 At the center the temperature may be up to 6 000 C 10 830 F 136 and the pressure could reach 360 GPa 52 million psi 137 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 138 139 The mean heat loss from Earth is 87 mW m 2 for a global heat loss of 4 42 1013 W 140 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 141 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 142 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 143 Magnetic field Main article Earth s magnetic field nbsp A schematic view of Earth s magnetosphere with solar wind flowing 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 144 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 145 146 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 147 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 148 Charged particles are contained within the magnetosphere the plasmasphere is defined by low energy particles that essentially follow magnetic field lines as Earth rotates 149 150 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 151 and the Van Allen radiation belts are formed by high energy particles whose motion is essentially random but contained in the magnetosphere 152 153 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 154 Orbit and rotationRotation Main article Earth s rotation nbsp Satellite time lapse imagery of Earth s rotation 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 155 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 156 157 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 2 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 2 Thus the sidereal day is shorter than the stellar day by about 8 4 ms 158 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 159 160 Orbit Main articles Earth s orbit and Earth s location nbsp 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 the 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 citation needed 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 citation needed 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 3 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 3 161 The Hill sphere or the sphere of gravitational influence of Earth is about 1 5 million km 930 000 mi in radius 162 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 162 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 163 Axial tilt and seasons Main article Axial tilt Earth nbsp 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 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 citation needed 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 164 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 165 166 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 167 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 168 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 169 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 170 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 171 Earth Moon systemFurther information Satellite system astronomy Moon Main articles Moon Lunar theory and Orbit of the Moon nbsp Earth and the Moon as seen from Mars by the Mars Reconnaissance Orbiter nbsp View of Earth from the Moon by the Lunar 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 although Charon is larger relative to the dwarf planet Pluto 172 173 The natural satellites of other planets are also referred to as moons after Earth s 174 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 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 The gravitational attraction between Earth and the Moon causes tides on Earth 175 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 176 As the Moon orbits Earth different parts of its face are illuminated by the Sun leading to the lunar phases 177 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 178 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 179 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 180 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 181 182 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 160 This allows total and annular solar eclipses to occur on Earth 183 On 1 November 2023 scientists reported that according to computer simulations remnants of a protoplanet named Theia could be inside the Earth left over from a collision with the Earth in ancient times and afterwards becoming the Moon 184 185 Asteroids and artificial satellites Main articles Near Earth object and Claimed moons of Earth nbsp 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 186 187 A trojan asteroid companion 2010 TK7 is librating around the leading Lagrange triangular point L4 in Earth s orbit around the Sun 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 190 There are also inoperative satellites including Vanguard 1 the oldest satellite currently in orbit and over 16 000 pieces of tracked space debris n 13 Earth s largest artificial satellite is the International Space Station 191 HydrosphereMain article Hydrosphere nbsp A view of Earth with its global ocean and cloud cover which dominate Earth s surface and hydrosphere at Earth s polar regions 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 down to a depth of 2 000 m 6 600 ft citation needed 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 192 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 193 About 97 5 of the water is saline the remaining 2 5 is fresh water 194 195 Most fresh water about 68 7 is present as ice in ice caps and glaciers 196 The remaining 30 is ground water 1 surface water covering only 2 8 of Earth s land 197 and other small forms of fresh water deposits such as permafrost water vapor in the atmosphere biological binding etc 198 199 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 200 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 201 The average salinity of Earth s oceans is about 35 grams of salt per kilogram of seawater 3 5 salt 202 Most of this salt was released from volcanic activity or extracted from cool igneous rocks 203 The oceans are also a reservoir of dissolved atmospheric gases which are essential for the survival of many aquatic life forms 204 Sea water has an important influence on the world s climate with the oceans acting as a large heat reservoir 205 Shifts in the oceanic temperature distribution can cause significant weather shifts such as the El Nino Southern Oscillation 206 The abundance of water particularly liquid 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 207 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 208 AtmosphereMain article Atmosphere of Earth nbsp A view of Earth with different layers of its atmosphere visible the troposphere with its clouds casting shadows a band of stratospheric blue sky at the horizon and a line of green airglow of the lower thermosphere 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 209 with a scale height of 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 209 Water vapor content varies between 0 01 and 4 209 but averages about 1 3 Clouds cover around two thirds of Earth s surface more so over oceans than land 210 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 211 Earth s biosphere has significantly altered its atmosphere Oxygenic photosynthesis evolved 2 7 Gya forming the primarily nitrogen oxygen atmosphere of today 62 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 212 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 213 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 214 and life on Earth probably would not exist in its current form 215 Weather and climate Main articles Weather and Climate nbsp The ITCZ s band of clouds over the Eastern Pacific and the Americas as seen from space nbsp Worldwide Koppen climate classifications Earth s atmosphere has no definite boundary gradually becoming thinner and fading into outer space 216 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 217 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 218 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 60 219 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 220 Earth receives 1361 W m2 of solar irradiance 221 222 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 223 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 224 Further factors that affect a location s climates are its proximity to oceans the oceanic and atmospheric circulation and topology 225 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 226 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 227 Finally temperatures decrease with height causing mountainous areas to be colder than low lying areas 228 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 218 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 229 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 219 The Koppen system rates regions based on observed temperature and precipitation 230 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 231 232 Upper atmosphere nbsp Earth s night side upper atmosphere appearing from the bottom as bands of afterglow illuminating the troposphere in orange with silhouettes of clouds and the stratosphere in white and blue Next the mesosphere pink area extends to the orange and faintly green line of the lowest airglow at about one hundred kilometers at the edge of space and the lower edge of the thermosphere invisible Continuing with green and red bands of aurorae streching over several hundred kilometers The upper atmosphere the atmosphere above the troposphere 233 is usually divided into the stratosphere mesosphere and thermosphere 213 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 234 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 235 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 236 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 237 Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth 238 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 239 Life on EarthMain articles Biosphere and History of life nbsp An animation of the changing density of productive vegetation 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 has ever been habitable for life Earth s life developed in Earth s early bodies of water some hundred million years after Earth formed citation needed Earth s life has been shaping and inhabiting many particular ecosystems on Earth and has eventually expanded globally forming an overarching biosphere 240 Therefore life has impacted Earth significantly altering Earth s atmosphere and surface over long periods of time causing changes like the Great Oxidation Event 241 Earth s life has over time greatly diversified allowing the biosphere to have different biomes which are inhabited by comparatively similar plants and animals 242 The different biomes developed 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 243 Earth provides liquid water an environment where complex organic molecules can assemble and interact and sufficient energy to sustain a metabolism 244 Plants and other organisms take up nutrients from water soils and the atmosphere These nutrients are constantly recycled between different species 245 nbsp A High Desert storm sweeps across the Mojave 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 246 Many places are subject to earthquakes landslides tsunamis volcanic eruptions tornadoes blizzards floods droughts wildfires and other calamities and disasters 247 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 248 Human activities release greenhouse gases into the atmosphere which cause global warming 249 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 250 Human geographyMain article Human geography See also World nbsp 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 251 In the 20th century Antarctica had been the last continent to see a first and until today limited human presence citation needed Human population has since the 19th century grown exponentially to seven billion in the early 2010s 252 and is projected to peak at around ten billion in the second half of the 21st century 253 Most of the growth is expected to take place in sub Saharan Africa 253 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 254 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 255 Furthermore since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century 256 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 Since the second half of the 20th century some hundreds of humans have temporarily stayed beyond Earth a tiny fraction of whom have reached another celestial body the Moon 257 258 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 by sovereign states countries separated by political borders and 205 such states exist today 259 with only parts of Antarctica and a few small regions remaining unclaimed 260 Most of these states together form the United Nations the leading worldwide intergovernmental organization 261 which extends human governance over the ocean and Antarctica and therefore all of Earth citation needed Natural resources and land use Main articles Natural resource and Land use nbsp Earth s land use for human agriculture Earth has resources that have been exploited by humans 262 Those termed non renewable resources such as fossil fuels are only replenished over geological timescales 263 Large deposits of fossil fuels are obtained from Earth s crust consisting of coal petroleum and natural gas 264 These deposits are used by humans both for energy production and as feedstock for chemical production 265 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 266 These metals and other elements are extracted by mining a process which often brings environmental and health damage 267 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 268 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 269 Of the 12 14 of ice free land that is used for croplands 2 percentage points were irrigated in 2015 270 Humans use building materials to construct shelters 271 Humans and the environment Main articles Human impact on the environment and Climate change nbsp Change in average surface air temperature and drivers for that change Human activity has caused increased temperatures with natural forces adding some variability 272 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 249 273 It is estimated that global temperatures in the year 2020 were 1 2 C 2 2 F warmer than the preindustrial baseline 274 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 250 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 275 276 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 277 Cultural and historical viewpointMain articles Earth in culture and Earth in science fiction nbsp Tracy Caldwell Dyson a NASA astronaut observing Earth from the Cupola module at the International Space Station on 11 September 2010 Human cultures have developed many views of the planet 278 The standard astronomical symbols of Earth are a quartered circle nbsp 279 representing the four corners of the world and a globus cruciger nbsp Earth is sometimes personified as a deity In many cultures it is a mother goddess that is also the primary fertility deity 280 Creation myths in many religions involve the creation of Earth by a supernatural deity or deities 280 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 281 282 283 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 284 285 In particular this caused a realization of the scope of effects from human activity on Earth s environment Enabled by science particularly Earth observation 286 humans have started to take action on environmental issues globally 287 acknowledging the impact of humans and the interconnectedness of Earth s environments citation needed 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 288 289 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 290 It was only during the 19th century that geologists realized Earth s age was at least many millions of years 291 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 292 293 See alsoCelestial sphere Earth phase 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 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 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 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 Source for minimum 19 mean 20 and maximum 21 surface temperature If Earth were shrunk to the size of a billiard ball some areas of Earth such as large mountain ranges and oceanic trenches 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