Platypus

Platypus[1]
Temporal range: 66–0 Ma
Late Cretaceous to Recent
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Monotremata
Family: Ornithorhynchidae
Genus: Ornithorhynchus
Blumenbach, 1800
Species: O. anatinus
Binomial name
Ornithorhynchus anatinus
(Shaw, 1799)
Platypus range
(blue — native, red — introduced)

The platypus (Ornithorhynchus anatinus) is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young. It is the sole living representative of its family (Ornithorhynchidae) and genus (Ornithorhynchus), though a number of related species have been found in the fossil record.

The unusual appearance of this egg-laying, venomous, duck-billed, beaver-tailed, otter-footed mammal baffled European naturalists when they first encountered it, with some considering it an elaborate fraud. It is one of the few venomous mammals, the male platypus having a spur on the hind foot that delivers a venom capable of causing severe pain to humans. The unique features of the platypus make it an important subject in the study of evolutionary biology and a recognisable and iconic symbol of Australia; it has appeared as a mascot at national events and is featured on the reverse of the Australian 20 cent coin. The platypus is the animal emblem of the state of New South Wales.[3]

Until the early 20th century, it was hunted for its fur, but it is now protected throughout its range. Although captive breeding programmes have had only limited success and the platypus is vulnerable to the effects of pollution, it is not under any immediate threat.

Contents

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Taxonomy and etymology

When the platypus was first encountered by Europeans in 1798, a pelt and sketch were sent back to Great Britain by Captain John Hunter, the second Governor of New South Wales.[4] British scientists' initial hunch was that the attributes were a hoax.[5] George Shaw, who produced the first description of the animal in the Naturalist's Miscellany in 1799, stated that it was impossible not to entertain doubts as to its genuine nature, and Robert Knox believed it might have been produced by some Asian taxidermist.[5] It was thought that somebody had sewn a duck's beak onto the body of a beaver-like animal. Shaw even took a pair of scissors to the dried skin to check for stitches.[6]

The common name "platypus" is the latinisation of the Greek word πλατύπους (platupous), "flat-footed",[7] from πλατύς (platus), "broad, wide, flat"[8] and πούς (pous), "foot".[9][10] Shaw assigned it as a Linnaean genus name when he initially described it, but the term was quickly discovered to belong already to the wood-boring ambrosia beetle (genus Platypus).[11] It was independently described as Ornithorhynchus paradoxus by Johann Blumenbach in 1800 (from a specimen given to him by Sir Joseph Banks)[12] and following the rules of priority of nomenclature it was later officially recognised as Ornithorhynchus anatinus.[11] The scientific name Ornithorhynchus anatinus is derived from ορνιθόρυνχος (ornithorhynkhos), which literally means "bird snout" in Greek, and anatinus, which means "duck-like" in Latin.

There is no universally agreed plural of "platypus" in the English language. Scientists generally use "platypuses" or simply "platypus". Colloquially, the term "platypi" is also used for the plural, although this is technically incorrect and a form of pseudo-Latin;[6] the correct Greek plural would be "platypodes". Early British settlers called it by many names, such as watermole, duckbill, and duckmole.[6] The name "platypus" is often prefixed with the adjective "duck-billed" to form duck-billed platypus, despite there being only one species of platypus.[13]

Description

The body and the broad, flat tail of the platypus are covered with dense, brown fur that traps a layer of insulating air to keep the animal warm.[6][11] The fur is waterproof, and the texture is akin to that of a mole.[14] The platypus uses its tail for storage of fat reserves (an adaptation also found in animals such as the Tasmanian devil[15] and fat-tailed sheep). It has webbed feet and a large, rubbery snout; these are features that appear closer to those of a duck than to those of any known mammal. The webbing is more significant on the front feet and is folded back when walking on land.[11] Unlike a bird's beak (in which the upper and lower parts separate to reveal the mouth), the snout of the platypus is a sensory organ with the mouth on the underside. The nostrils are located on the dorsal surface of the snout, while the eyes and ears are located in a groove set just back from it; this groove is closed when swimming.[11] Platypuses have been heard to emit a low growl when disturbed and a range of other vocalisations have been reported in captive specimens.[6]

A colour print of platypuses from 1863

Weight varies considerably from 0.7 to 2.4 kg (1.5 to 5.3 lb), with males being larger than females: males average 50 cm (20 in) in total length while females average 43 cm (17 in).[11] There is substantial variation in average size from one region to another, and this pattern does not seem to follow any particular climatic rule and may be due to other environmental factors, such as predation and human encroachment.[16]

The platypus has an average body temperature of about 32 °C (90 °F) rather than the 37 °C (99 °F) typical of placental mammals.[17] Research suggests this has been a gradual adaptation to harsh environmental conditions on the part of the small number of surviving monotreme species rather than a historical characteristic of monotremes.[18][19]

Modern platypus young have three-cusped molars, which they lose before or just after leaving the breeding burrow;[20][21] adults have heavily keratinised pads in their place.[11] The platypus jaw is constructed differently from that of other mammals, and the jaw-opening muscle is different.[11] As in all true mammals, the tiny bones that conduct sound in the middle ear are fully incorporated into the skull, rather than lying in the jaw as in cynodonts and other premammalian synapsids. However, the external opening of the ear still lies at the base of the jaw.[11] The platypus has extra bones in the shoulder girdle, including an interclavicle, which is not found in other mammals.[11] It has a reptilian gait, with legs that are on the sides of the body, rather than underneath.[11] When on land, it engages in knuckle-walking to protect the webbing between its toes.[22]

Venom

The calcaneus spur found on the male's hind limb is used to deliver venom.

While both male and female platypuses are born with ankle spurs, only the male has spurs which produce a cocktail of venom,[23][24][25] composed largely of defensin-like proteins (DLPs), three of which are unique to the platypus.[26] The defensin proteins are produced by the immune system of the platypus. Although powerful enough to kill smaller animals such as dogs, the venom is not lethal to humans, but is so excruciating that the victim may be incapacitated.[26][27] Oedema rapidly develops around the wound and gradually spreads throughout the affected limb. Information obtained from case histories and anecdotal evidence indicates the pain develops into a long-lasting hyperalgesia (a heightened sensitivity to pain) that persists for days or even months.[28][29] Venom is produced in the crural glands of the male, which are kidney-shaped alveolar glands connected by a thin-walled duct to a calcaneus spur on each hind limb. The female platypus, in common with echidnas, has rudimentary spur buds which do not develop (dropping off before the end of their first year) and lack functional crural glands.[11]

The venom appears to have a different function from those produced by nonmammalian species: its effects are not life-threatening to humans, but nevertheless powerful enough to seriously impair the victim. Since only males produce venom and production rises during the breeding season, it may be used as an offensive weapon to assert dominance during this period.[26]

Electrolocation

Platypus (Ornithorhynchus anatinus)

Monotremes (for the other species, see Echidna) are the only mammals known to have a sense of electroreception: they locate their prey in part by detecting electric fields generated by muscular contractions. The platypus' electroreception is the most sensitive of any monotreme.[30][31]

The electroreceptors are located in rostrocaudal rows in the skin of the bill, while mechanoreceptors (which detect touch) are uniformly distributed across the bill. The electrosensory area of the cerebral cortex is contained within the tactile somatosensory area, and some cortical cells receive input from both electroreceptors and mechanoreceptors, suggesting a close association between the tactile and electric senses. Both electroreceptors and mechanoreceptors in the bill dominate the somatotopic map of the platypus brain, in the same way human hands dominate the Penfield homunculus map.[32][33]

The platypus can determine the direction of an electric source, perhaps by comparing differences in signal strength across the sheet of electroreceptors. This would explain the characteristic side-to-side motion of the animal's head while hunting. The cortical convergence of electrosensory and tactile inputs suggests a mechanism for determining the distance of prey items which, when they move, emit both electrical signals and mechanical pressure pulses; the difference between the times of arrival of the two signals would allow computation of distance.[31]

The platypus feeds by neither sight nor smell,[34] closing its eyes, ears, and nose each time it dives.[35] Rather, when it digs in the bottom of streams with its bill, its electroreceptors detect tiny electrical currents generated by muscular contractions of its prey, so enabling it to distinguish between animate and inanimate objects, which continuously stimulate its mechanoreceptors.[31] Experiments have shown the platypus will even react to an "artificial shrimp" if a small electrical current is passed through it.[36]

Eyes

Recent studies say that the eyes of the platypus could possibly be highly similar to those of a Pacific hagfish or North Hemisphere Lampreys and to those of most tetrapods. Also it contains double cones, which most mammals do not have. [37]

Ecology and behaviour

Dentition, as illustrated in Knight's Sketches in Natural History
The platypus is very difficult to spot even on the surface of a river.
Platypus swimming
Ornithorhynchus anatinus -Sydney Aquarium, Sydney, Australia -swimming-6a.ogv
Swimming underwater at Sydney Aquarium, Australia

The platypus is semiaquatic, inhabiting small streams and rivers over an extensive range from the cold highlands of Tasmania and the Australian Alps to the tropical rainforests of coastal Queensland as far north as the base of the Cape York Peninsula.[38] Inland, its distribution is not well known: it is extinct in South Australia (apart from an introduced population on Kangaroo Island)[39] and is no longer found in the main part of the Murray-Darling Basin, possibly due to the declining water quality brought about by extensive land clearing and irrigation schemes.[40] Along the coastal river systems, its distribution is unpredictable; it appears to be absent from some relatively healthy rivers, and yet maintains a presence in others that are quite degraded (the lower Maribyrnong, for example).[41]

In captivity, platypuses have survived to 17 years of age, and wild specimens have been recaptured when 11 years old. Mortality rates for adults in the wild appear to be low.[11] Natural predators include snakes, water rats, goannas, hawks, owls, and eagles. Low platypus numbers in northern Australia are possibly due to predation by crocodiles.[42] The introduction of red foxes in 1845 for hunting may have had some impact on its numbers on the mainland.[16] The platypus is generally regarded as nocturnal and crepuscular, but individuals are also active during the day, particularly when the sky is overcast.[43][44] Its habitat bridges rivers and the riparian zone for both a food supply of prey species, and banks where it can dig resting and nesting burrows.[44] It may have a range of up to 7 km (4.3 mi), with a male's home range overlapping those of three or four females.[45]

The platypus is an excellent swimmer and spends much of its time in the water foraging for food. When swimming, it can be distinguished from other Australian mammals by the absence of visible ears.[46] Uniquely among mammals, it propels itself when swimming by an alternate rowing motion of the front two feet; although all four feet of the platypus are webbed, the hind feet (which are held against the body) do not assist in propulsion, but are used for steering in combination with the tail.[47] The species is endothermic, maintaining its body temperature at about 32 °C (90 °F), lower than most mammals, even while foraging for hours in water below 5 °C (41 °F).[11]

Dives normally last around 30 seconds, but can last longer, although few exceed the estimated aerobic limit of 40 seconds. Recovery at the surface between dives commonly takes from 10 to 20 seconds.[48][49] The platypus is a carnivore: it feeds on annelid worms and insect larvae, freshwater shrimps, and yabbies (freshwater crayfish) that it digs out of the riverbed with its snout or catches while swimming. It uses cheek-pouches to carry prey to the surface, where they are eaten.[46] The platypus needs to eat about 20% of its own weight each day, which requires it to spend an average of 12 hours each day looking for food.[48] When not in the water, the platypus retires to a short, straight resting burrow of oval cross-section, nearly always in the riverbank not far above water level, and often hidden under a protective tangle of roots.[46]

The average sleep time of a platypus is said to be as long as 14 hours per day, possibly because they eat crustaceans which provide a high level of calories.[50]

Reproduction

When the platypus was first encountered by European naturalists, they were divided over whether the female laid eggs. This was not confirmed until 1884, when W. H. Caldwell was sent to Australia, where, after extensive searching assisted by a team of 150 Aborigines, he managed to discover a few eggs.[11][26] Mindful of the high cost per word of wiring England, Caldwell famously but tersely wired London, "Monotremes oviparous, ovum meroblastic." That is, monotremes lay eggs, and the eggs are similar to those of reptiles in that only part of the egg divides as it develops.

The species exhibits a single breeding season; mating occurs between June and October, with some local variation taking place between different populations across its range.[42] Historical observation, mark-and-recapture studies, and preliminary investigations of population genetics indicate the possibility of both resident and transient members of populations, and suggest a polygynous mating system.[51] Females are thought likely to become sexually mature in their second year, with breeding confirmed still to take place in animals over nine years old.[51]

Outside the mating season, the platypus lives in a simple ground burrow whose entrance is about 30 cm (12 in) above the water level. After mating, the female constructs a deeper, more elaborate burrow up to 20 m (66 ft) long and blocked at intervals with plugs (which may act as a safeguard against rising waters or predators, or as a method of regulating humidity and temperature).[52] The male takes no part in caring for its young, and retreats to its year-long burrow. The female softens the ground in the burrow with dead, folded, wet leaves and she fills the nest at the end of the tunnel with fallen leaves and reeds for bedding material. This material is dragged to the nest by tucking it underneath her curled tail.[6]

The female platypus has a pair of ovaries, but only the left one is functional.[43] It lays one to three (usually two) small, leathery eggs (similar to those of reptiles), about 11 mm (0.43 in) in diameter and slightly rounder than bird eggs.[53] The eggs develop in utero for about 28 days, with only about 10 days of external incubation (in contrast to a chicken egg, which spends about one day in tract and 21 days externally).[43] After laying her eggs, the female curls around them. The incubation period is divided into three phases. In the first phase, the embryo has no functional organs and relies on the yolk sac for sustenance. The yolk is absorbed by the developing young.[54] During the second phase, the digits develop and, in the last phase, the egg tooth appears.[55]

The newly hatched young are vulnerable, blind, and hairless, and are fed by the mother's milk. Although possessing mammary glands, the platypus lacks teats. Instead, milk is released through pores in the skin. There are grooves on her abdomen in which the milk pools, allowing the young to lap it up.[6][42] After they hatch, the offspring are suckled for three to four months. During incubation and weaning, the mother initially leaves the burrow only for short periods, to forage. When doing so, she creates a number of thin soil plugs along the length of the burrow, possibly to protect the young from predators; pushing past these on her return forces water from her fur and allows the burrow to remain dry.[56] After about five weeks, the mother begins to spend more time away from her young and, at around four months, the young emerge from the burrow.[42] A platypus is born with teeth, but these drop out at a very early age, leaving the horny plates with which it grinds its food.[57]

Evolution

Platypus skeleton

The platypus and other monotremes were very poorly understood, and some of the 19th century myths that grew up around them—for example, that the monotremes were "inferior" or quasireptilian—still endure.[58] In 1947, William King Gregory theorised that placental mammals and marsupials may have diverged earlier, and a subsequent branching divided the monotremes and marsupials, but later research and fossil discoveries have suggested this is incorrect.[58][59] In fact, modern monotremes are the survivors of an early branching of the mammal tree, and a later branching is thought to have led to the marsupial and placental groups.[58][60] Molecular clock and fossil dating suggest platypuses split from echidnas around 19–48 million years ago.[61]




Platypus



Echidnas



live birth

Marsupials


true placenta

Eutherians




Evolutionary relationships between the platypus and other mammals.[62]

The oldest discovered fossil of the modern platypus dates back to about 100,000 years ago, during the Quaternary period. The extinct monotremes Teinolophos and Steropodon were closely related to the modern platypus.[59] The fossilised Steropodon was discovered in New South Wales and is composed of an opalised lower jawbone with three molar teeth (whereas the adult contemporary platypus is toothless). The molar teeth were initially thought to be tribosphenic, which would have supported a variation of Gregory's theory, but later research has suggested that, while they have three cusps, they evolved under a separate process.[20] The fossil is thought to be about 110 million years old, which means that the platypus-like animal was alive during the Cretaceous period, making it the oldest mammal fossil found in Australia. Monotrematum sudamericanum, another fossil relative of the platypus, has been found in Argentina, indicating monotremes were present in the supercontinent of Gondwana when the continents of South America and Australia were joined via Antarctica (up to about 167 million years ago).[20][63]

Because of the early divergence from the therian mammals and the low numbers of extant monotreme species, the platypus is a frequent subject of research in evolutionary biology. In 2004, researchers at the Australian National University discovered the platypus has ten sex chromosomes, compared with two (XY) in most other mammals (for instance, a male platypus is always XYXYXYXYXY),[64] although given the XY designation of mammals, the sex chromosomes of the platypus are more similar to the ZZ/ZW sex chromosomes found in birds.[65] The platypus genome also has both reptilian and mammalian genes associated with egg fertilisation.[66] Since the platypus lacks the mammalian sex-determining gene SRY, the mechanism of sex determination remains unknown.[67] A draft version of the platypus genome sequence was published in Nature on 8 May 2008, revealing both reptilian and mammalian elements, as well as two genes found previously only in birds, amphibians, and fish. More than 80% of the platypus' genes are common to the other mammals whose genomes have been sequenced.[66]

Conservation status

A depiction of a platypus from a book for children published in Germany in 1798

Except for its loss from the state of South Australia, the platypus occupies the same general distribution as it did prior to European settlement of Australia. However, local changes and fragmentation of distribution due to human modification of its habitat are documented. Its current and historical abundance, however, are less well-known and it has probably declined in numbers, although still being considered as common over most of its current range.[44] The species was extensively hunted for its fur until the early years of the 20th century and, although protected throughout Australia since 1905,[56] until about 1950 it was still at risk of drowning in the nets of inland fisheries.[40] The platypus does not appear to be in immediate danger of extinction thanks to conservation measures, but it could be impacted by habitat disruption caused by dams, irrigation, pollution, netting, and trapping.[2] The IUCN lists the platypus on its Red List as Least Concern.[2]

Platypuses generally suffer from few diseases in the wild; however, there is widespread public concern in Tasmania about the potential impacts of a disease caused by the fungus Mucor amphibiorum. The disease (termed mucormycosis) affects only Tasmanian platypuses, and has not been observed in platypuses in mainland Australia. Affected platypuses can develop ugly skin lesions or ulcers on various parts of the body, including their backs, tails, and legs. Mucormycosis can kill platypuses, death arising from secondary infection and by affecting the animals' ability to maintain body temperature and forage efficiency. The Biodiversity Conservation Branch at the Department of Primary Industries and Water are collaborating with NRM north and University of Tasmania researchers to determine the impacts of the disease on Tasmanian platypuses, as well as the mechanism of transmission and current spread of the disease.[68] Until recently, the introduced red fox (Vulpes vulpes) was confined to mainland Australia, but growing evidence now indicates it is present in low numbers in Tasmania.[69]

Much of the world was introduced to the platypus in 1939 when National Geographic Magazine published an article on the platypus and the efforts to study and raise it in captivity. The latter is a difficult task, and only a few young have been successfully raised since—notably at Healesville Sanctuary in Victoria. The leading figure in these efforts was David Fleay, who established a platypusary—a simulated stream in a tank—at the Healesville Sanctuary, where breeding was successful in 1943. In 1972, he found a dead baby of about 50 days old, which had presumably been born in captivity, at his wildlife park at Burleigh Heads on the Gold Coast, Queensland.[70] Healesville repeated its success in 1998 and again in 2000 with a similar stream tank. Taronga Zoo in Sydney bred twins in 2003, and breeding was again successful there in 2006.[71]

Platypus in wildlife sanctuaries

Platypus House at Lone Pine Koala Sanctuary in Brisbane, Queensland

The platypus can be seen in special aquariums at the following Australian wildlife sanctuaries:

Queensland

Gold Coast

Brisbane

New South Wales

Sydney

Victoria

Healesville

South Australia

Mylor

Cultural references

The Australian 20 cent coin features a platypus.

Since the introduction of decimal currency to Australia in 1966, the embossed image of a platypus has appeared on the reverse (tail) side of the 20 cent coin.

The platypus has been used several times as a mascot: "Syd" the platypus was one of the three mascots chosen for the Sydney 2000 Olympics along with an echidna and a kookaburra,[73] "Expo Oz" the platypus was the mascot for World Expo 88, which was held in Brisbane in 1988,[74] and Hexley the platypus is the mascot for Apple Computer's BSD-based Darwin operating system, Mac OS X.[75] The platypus is also the mascot for the currently inactive Wenatchee Valley Venom arena football team located in Wenatchee, Washington.

The Platypus Trophy was made as an award for the winner of the college rivalry between the Oregon Ducks and the Oregon State Beavers.

The platypus has also been featured in songs, such as Green Day's "Platypus (I Hate You)" and Mr. Bungle's "Platypus". It is the subject of a children's poem by Banjo Paterson,[76] and it also frequently appears as a character in children's television programmes, for example, the Platypus Family on Mister Rogers' Neighborhood, Perry the Platypus on the show Phineas and Ferb, and Ovide, the star of the cartoon Ovide and the Gang.[77]

In the 1980s, the platypus was the main animal featured on promotional ads for the educational animal encyclopedia "Wildlife Treasury". In the advertisement, an excited young boy exclaims: "The Duck Billed Platypus has feet like a duck but it's furry! It's all in my Wildlife Treasury!"

The platypus is sometimes jokingly referred to as proof that God has a sense of humour (at the beginning of the film Dogma, for example; Robin Williams implied he was also stoned on marijuana at the time.[78]) It is also often used humorously (along with the camel) to describe something designed by committee. For the "Star Trek: The Next Generation" novel Q-Squared, the titled character claims (in private) to a disbelieving Capt. Picard he personally influenced God's decision to create/evolve the platypus.

The platypus is also a pet in the massively multiplayer online role playing game RuneScape.

"Platypus Man" was a short lived, 1995 sitcom aired on Fox Television and/or UPN in the United States. Fox also had an animated program called "Taz-Mania," featuring the Platypus Bros.: Daniel and Timothy.

In the episode, The Truth in the Myth on Bones, when the team was studying a murdered victim who was supposed to be killed by the cryptic animal, chupacabra, Dr. Saroyan questions the very existence of such a creature, in which intern Vincent Nigel-Murray responded, "I'd be skeptical if you told me there was a venomous, egg-laying, duck-billed, beaver-tailed mammal, and yet the platypus, it does exist."

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Earth

Earth Astronomical symbol of Earth
A planetary disk of white cloud formations, brown and green land masses, and dark blue oceans against a black background. The Arabian peninsula, Africa and Madagascar lie in the upper half of the disk, while Antarctica is at the bottom.
"The Blue Marble" photograph of Earth,
taken from Apollo 17
Designations
Alternate name(s)Terra
Epoch J2000.0[note 1]
Aphelion152,098,232 km
1.01671388 AU[note 2]
Perihelion147,098,290 km
0.98329134 AU[note 2]
Semi-major axis149,598,261 km
1.00000261 AU[1]
Eccentricity0.01671123[1]
Orbital period365.256363004 days[2]
1.000017421 yr
Average orbital speed29.78 km/s[3]
107,200 km/h
Mean anomaly357.51716°[3]
Inclination7.155° to Sun's equator
1.57869°[4] to invariable plane
Longitude of ascending node348.73936°[3][note 3]
Argument of perihelion114.20783°[3][note 4]
Satellites1 natural (The Moon)
8,300+ artificial (as of 1 March 2001)[5]
Physical characteristics
Mean radius6,371.0 km[6]
Equatorialradius6,378.1 km[7][8]
Polar radius6,356.8 km[9]
Flattening0.0033528[10]
Circumference40,075.017 km (equatorial)[8]
40,007.86 km (meridional)[11]
Surface area510,072,000 km2[12][13][note 5]

148,940,000 km2 land (29.2 %)

361,132,000 km2 water (70.8 %)
Volume1.08321×1012 km3[3]
Mass5.9736×1024 kg[3]
Mean density5.515 g/cm3[3]
Equatorial surface gravity9.780327 m/s2[14]
0.99732 g
Escape velocity11.186 km/s[3]
Sidereal rotation
period
0.99726968 d[15]
23h 56m 4.100s
Equatorial rotation velocity1,674.4 km/h (465.1 m/s)[16]
Axial tilt23°26'21".4119[2]
Albedo0.367 (geometric)[3]
0.306 (Bond)[3]
Surface temp.
Kelvin
Celsius
minmeanmax
184 K[17]287.2 K[18]331 K[19]
−89.2 °C14 °C57.8 °C
Atmosphere
Surfacepressure101.325 kPa (MSL)
Composition78.08% nitrogen (N2)[3]
20.95% oxygen (O2)
0.93% argon
0.038% carbon dioxide
About 1% water vapor (varies with climate)

Earth (or the Earth) is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world, the Blue Planet,[20] or by its Latin name, Terra.[note 6]

Earth formed 4.54 billion years ago, and life appeared on its surface within one billion years.[21] The planet is home to millions ofspecies, including humans.[22] Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar radiation, permitting life on land.[23] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life for another 500 million to2.3 billion years.[24][25][26]

Earth's outer surface is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap). The planet's interior remains active, with a thick layer of relatively solidmantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth orbits the Sun once every 366.26 times it rotates about its own axis, which is equal to 365.26 solar days, or one sidereal year.[note 7] The Earth's axis of rotation is tilted23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).[27] Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation. Between approximately 3.8 billion and 4.1 billionyears ago, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment.

Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population.[28] These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a flat Earth or in the Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.

Contents

[hide]

Chronology

The earliest dated Solar System material was formed 4.5672 ± 0.0006 billion years ago,[29] and by 4.54 billion years ago (within an uncertainty of 1%)[21] the Earth and the other planets in the Solar System had formed out of the solar nebula—a disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was thus largely completed within 10–20 million years.[30] Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed shortly thereafter, 4.53 billion years ago.[31]

The current consensus model[32] for the formation of the Moon is the giant impact hypothesis, in which the Moon was created when a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass[33] impacted the Earth in a glancing blow.[34] In this model, some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to coalesce into the Moon.

Outgassing and volcanic activity produced the primordial atmosphere of the Earth. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans.[35] The newly formed Sun was only 70% of its present luminosity, yet evidence shows that the early oceans remained liquid—a contradiction dubbed the faint young Sun paradox. A combination of greenhouse gases and higher levels of solar activity served to raise the Earth's surface temperature, preventing the oceans from freezing over.[36] By 3.5 billion years ago, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[37]

Two major models have been proposed for the rate of continental growth:[38] steady growth to the present-day[39] and rapid growth early in Earth history.[40] Current research shows that the second option is most likely, with rapid initial growth of continental crust[41] followed by a long-term steady continental area.[42][43][44] On time scales lasting hundreds of millions of years, the surface continually reshaped as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly750 million years ago (Ma), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 Ma, then finallyPangaea, which broke apart 180 Ma.[45]

Evolution of life

The general hypothesis is that highly energetic chemistry produced a self-replicating molecule around 4 billion years ago and half a billion years later the last common ancestor of all lifeexisted.[46] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer ofozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[47]True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[48]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[49]

Following the Cambrian explosion, about 535 Ma, there have been five major mass extinctions.[50] The most recent such event was 65 Ma, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.[51] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[52] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 Ma and then intensified during the Pleistocene about 3 Ma. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago.[53]

Future

14 billion year timeline showing Sun's present age at 4.6 billion years; from 6 billion years Sun gradually warming, becoming a red dwarf at 10 billion years, "soon" followed by its transformation into a white dwarf star
The life cycle of the Sun

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 Gyr (1.1 billion years) and by 40% over the next 3.5 Gyr.[54] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the loss of the planet's oceans.[55]

The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 500 million[24] to 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.[56] After another billion years all surface water will have disappeared[25] and the mean global temperature will reach 70 °C[56] (158 °F). The Earth is expected to be effectively habitable for about another 500 million years from that point,[24] although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere.[26] Even if the Sun were eternal and stable, the continued internal cooling of the Earth would result in a loss of much of its CO2 due to reduced volcanism,[57] and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.[58]

The Sun, as part of its evolution, will become a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km).[54][59] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. The planet was therefore initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5000 times its present level).[54] A 2008 simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized.[59]

Composition and structure

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

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation,[60] and is probably the only one with active plate tectonics.[61]

Shape

Chimborazo, Ecuador. The furthermost point on the Earth's surface from its center.[62]

The shape of the Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.[63] This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km larger than the pole-to-pole diameter.[64] For this reason the furthest point on the surface from the Earth's center of mass is the Chimborazo volcano in Ecuador.[65]The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[66]

Local topography deviates from this idealized spheroid, although on a global scale, these deviations are small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls.[67] The largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.[68][69][70]

Chemical composition of the crust[71]
CompoundFormulaComposition
ContinentalOceanic
silicaSiO260.2%48.6%
aluminaAl2O315.2%16.5%
limeCaO5.5%12.3%
magnesiaMgO3.1%6.8%
iron(II) oxideFeO3.8%6.2%
sodium oxideNa2O3.0%2.6%
potassium oxideK2O2.8%0.4%
iron(III) oxideFe2O32.5%2.3%
waterH2O1.4%1.1%
carbon dioxideCO21.2%1.4%
titanium dioxideTiO20.7%1.4%
phosphorus pentoxideP2O50.2%0.3%
Total99.6%99.9%

Chemical composition

The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon(15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is estimated to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[72]

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right), with the other constituents occurring in minute quantities.[73]

Internal structure

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer of the Earth 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, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. 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 kilometers 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.[74] The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[75]

Geologic layers of the Earth[76]
Earth-crust-cutaway-english.svg

Earth cutaway from core to exosphere. Not to scale.
Depth[77]
km
Component LayerDensity
g/cm3
0–60Lithosphere[note 8]
0–35Crust[note 9]2.2–2.9
35–60Upper mantle3.4–4.4
35–2890Mantle3.4–5.6
100–700Asthenosphere
2890–5100Outer core9.9–12.2
5100–6378Inner core12.8–13.1

Heat

Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[78] The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[79] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[80] Because much of the heat is provided by radioactive decay, scientists surmise that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately 3 billion years ago,[78] would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.[81]

Present-day major heat-producing isotopes[82]
IsotopeHeat release
W/kg isotope
Half-life

years
Mean mantle concentration
kg isotope/kg mantle
Heat release
W/kg mantle
238U9.46 × 10−54.47 × 10930.8 × 10−92.91 × 10−12
235U5.69 × 10−47.04 × 1080.22 × 10−91.25 × 10−13
232Th2.64 × 10−51.40 × 1010124 × 10−93.27 × 10−12
40K2.92 × 10−51.25 × 10936.9 × 10−91.08 × 10−12

The mean heat loss from the Earth is 87 mW m−2, for a global heat loss of 4.42 × 1013 W.[83] 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.[84] More of the heat in the 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 in the oceans because the crust there is much thinner than that of the continents.[85]

Tectonic plates

Earth's main plates[86]
Shows the extent and boundaries of tectonic plates, with superimposed outlines of the continents they support
Plate nameArea
106 km2
103.3
78.0
75.9
67.8
60.9
47.2
43.6

The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[87] 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,[88] and their motion is strongly coupled with convection patterns inside the Earth's mantle.

As the tectonic plates migrate across the planet, the ocean floor 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 continually recycles the oceanic crust back into the mantle. Because of this recycling, most of the ocean floor is less than 100 million years in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 millionyears.[89][90] By comparison, the oldest dated continental crust is 4030 million years old.[91]

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 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/year[92] and the Pacific Plate moving 52–69 mm/year. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/year.[93]

Surface

The Earth's terrain varies greatly from place to place. About 70.8%[94] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[64] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods because of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[95] also act to reshape the landscape.

The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[96] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.[97] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.[98]Common carbonate minerals include calcite (found in limestone) and dolomite.[99]

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.[13] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3×107 km2 of cropland and 3.4×107 km2 of pastureland.[100]

The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.[101]

Hydrosphere

Elevation histogram of the surface of the Earth

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location isChallenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m.[note 11][102]

The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3.[103] If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[note 12] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.[104]

The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (35 ).[105] Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[106] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[107] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[108] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[109]

Atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.[3] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation resulting from weather and seasonal factors.[110]

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[111] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface would be −18 °C, in contrast to the current +15 °C, and life would likely not exist.[94]

Weather and climate

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.[112]

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°.[113] Ocean currents are also important factors in determining climate, particularly the thermohaline circulationthat distributes heat energy from the equatorial oceans to the polar regions.[114]

Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.[112] 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, topological features and temperature differences determine the average precipitation that falls in each region.[115]

The amount of solar energy reaching the Earth's decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at a 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 per per degree of latitude away from the equator.[116] The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical,temperate and polar climates.[117] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes,continental and cold polar), which are further divided into more specific subtypes.[113]

Upper atmosphere

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

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[111] Each layer has a differentlapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the Earth's magnetic fields interact with the solar wind.[118] 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 above the Earth's surface, is a working definition for the boundary between atmosphere and space.[119]

Thermal energy causes some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can escape from the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogenhas a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses.[120]The leakage of hydrogen into space contributes to the pushing of the Earth 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 presumed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[121] Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.[122] 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.[123]

Magnetic field

Diagram showing the magnetic field lines of the Earth's magnetosphere. The lines are swept back in the anti-solar direction under the influence of the solar wind.
Schematic of Earth's magnetosphere. The solar windflows from left to right

The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. At the equator of the magnetic field, the magnetic field strength at the planet's surface is 3.05 × 10−5 T, with global magnetic dipole moment of 7.91 × 1015 T m3.[124] According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[125][126]

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.[127]

Orbit and rotation

Rotation

Earth's axial tilt (or obliquity) and its relation to therotation axis and plane of orbit

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).[128]As the Earth's solar day is now slightly longer than it was during the 19th century because of tidal acceleration, each day varies between 0 and 2 SI ms longer.[129][130]

Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86164.098903691 seconds of mean solar time (UT1), or 23h 56m 4.098903691s.[2][note 13] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s).[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[131] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[132] and 1962–2005.[133]

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the 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 Moon every two minutes; from the planet's surface, the apparent sizes of the Sun and the Moon are approximately the same.[134][135]

Orbit

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter, every 12 hours. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.[3]

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.4 degrees from the perpendicular to the Earth–Sun plane, and the Earth–Moon plane is tilted about 5 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][136]

The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius.[137][note 14] This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Barred spiral galaxy
Illustration of the Milky Way Galaxy, showing the location of the Sun

Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 light years from the center of the galaxy. It is currently about 20 light years above the galaxy's equatorial plane in the Orion spiral arm.[138]

Axial tilt and seasons

Because of the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This results in seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.

Black space with crescent Earth at lower left, crescent Moon at upper right, 30% of Earth's apparent diameter; five Earth diameters apparent space between; sunlit from right side
Earth and Moon from Mars, imaged byMars Reconnaissance Orbiter. From space, the Earth can be seen to go through phases similar to the phases of the Moon.

By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, Spring Equinox is around March 20 andAutumnal Equinox is about September 23. In the Southern hemisphere, the situation is reversed, with the Summer and Winter Solstices exchanged and the Spring and Autumnal Equinox dates switched.[139]

The angle of the Earth's tilt is relatively stable over long periods of time. The tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.[140] The orientation (rather than the angle) of the 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 Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the 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. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.[141]

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4. These dates change over time due to precessionand other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance results in an increase of about 6.9%[note 15] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the 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.[142]

Moon

Characteristics
Diameter3,474.8 km
Mass7.349×1022 kg
Semi-major axis384,400 km
Orbital period27 d 7 h 43.7 m

The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the 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. The natural satellites orbiting other planets are called "moons" after Earth's Moon.

The gravitational attraction between the Earth and Moon causes tides on Earth. 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 the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.

Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs a year—add up to significant changes.[143] During the Devonian period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.[144]

Details of the Earth-Moon system. Besides the radius of each object, the radius to the Earth-Moon barycenter is shown. Photos from NASA. Data from NASA. The Moon's axis is located by Cassini's third law.

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.[145] Some theorists think that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.[146]

Viewed from Earth, the Moon is just far enough away to have very nearly 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.[135] This allows total and annular solar eclipses to occur on Earth.

The most widely accepted theory of the Moon's origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.[147]

Earth has at least five co-orbital asteroids, including 3753 Cruithne and 2002 AA29.[148][149] As of 2011, there are 931 operational, man-made satellites orbiting the Earth.[150] On July 27, 2011, astronomers reported a trojan asteroid companion,2010 TK7, librating around the leading Lagrange triangular point, L4, of Earth in Earth's orbit around the Sun.[151][152]

A scale representation of the relative sizes of, and average distance between, Earth and Moon

Habitability

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.[153] The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the current climactic conditions at the surface.[154]

Biosphere

The planet's life forms are sometimes said to form a "biosphere". The general hypothesis is that this biosphere had begun evolving about 3.5 billion years ago. The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land, biomes are separated primarily by differences in latitude, height above sea level and humidity. Terrestrial biomes lying within the Arctic or Antarctic Circles, at high altitudes or in extremely arid areas are relatively barren of plant and animal life; species diversity reaches a peak inhumid lowlands at equatorial latitudes.[155]

Natural resources and land use

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.

Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics.[156] These bodies form concentrated sources for many metals and other useful elements.

The Earth's biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.[157] Humans also live on the land by using building materials to construct shelters. In 1993, human use of land is approximately:

Land useArable landPermanent cropsPermanent pasturesForests and woodlandUrban areasOther
Percentage13.13%[13]4.71%[13]26%32%1.5%30%

The estimated amount of irrigated land in 1993 was 2,481,250 km2.[13]

Natural and environmental hazards

Large areas of the Earth's surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980–2000, these events caused an average of 11,800 deaths per year.[158] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts,wildfires, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.

According to the United Nations, a scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.[159]

Human geography

Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Earth has reached approximately 7,000,000,000 human inhabitants as of October 31, 2011.[160] Projections indicate that the world's human population will reach 9.2 billion in 2050.[161]Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.[162]

It is estimated that only one-eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is either desert (14%),[163]high mountains (27%),[164] or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[165] (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)

Independent sovereign nations claim the planet's entire land surface, except for some parts of Antarctica and the odd unclaimed area of Bir Tawil between Egypt and Sudan. As of 2011 there are 204 sovereign states, including the 193 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities.[13] Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.[166]

The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict.[167] The U.N. serves primarily as a forum for international diplomacy and international law. When the consensus of the membership permits, it provides a mechanism for armed intervention.[168]

The first human to orbit the Earth was Yuri Gagarin on April 12, 1961.[169] In total, about 487 people have visited outer space and reached Earth orbit as of July 30, 2010, and, of these,twelve have walked on the Moon.[170][171][172] Normally the only humans in space are those on the International Space Station. The station's crew, currently six people, is usually replaced every six months.[173] The furthest humans have travelled from Earth is 400,171 km, achieved during the 1970 Apollo 13 mission.[174]

The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.
Northwest coast of United States to Central South America at Night.ogv
ISS video beginning just south-east of Alaska. The first city that the ISS passes over (seen approximately 10 seconds into the video) is San Francisco and the surrounding areas. If one looks very carefully, you can spot where the Golden Gate Bridge is located: a smaller strip of lights just before the city of San Francisco, nearest to the clouds on the right of the image. Very obviouslightning storms can be seen on the Pacific Oceancoastline, with clouds overhead. As the video continues, the ISS passes over Central America(green lights can be seen here), with the Yucatan Peninsula on the left. The pass ends as the ISS is over the capital city of Bolivia, La Paz.

Cultural viewpoint

The first photograph ever taken by astronauts of an "Earthrise", from Apollo 8

The name "Earth" derives from the Anglo-Saxon word erda, which means ground or soil, and is related to the German word Erde. It becameeorthe later, and then erthe in Middle English.[176] The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.[177]

Unlike the rest of the planets in the Solar System, humankind did not begin to view the Earth as a moving object in orbit around the Sun until the 16th century.[178] Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism[179] or Islam,[180] assert that theirinterpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life.[181] Such assertions are opposed by the scientific community[182][183] and by other religious groups.[184][185][186] A prominent example is the creation-evolution controversy.

In the past there were varying levels of belief in a flat Earth,[187] but this was displaced by the concept of a spherical Earth due to observation and circumnavigation.[188] The human perspective regarding the Earth has changed following the advent of spaceflight, and the biosphere is now widely viewed from a globally integrated perspective.[189][190] This is reflected in a growing environmental movement that is concerned about humankind's effects on the planet.[191]

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