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 normal 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.
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 SpanishTierra and the FrenchTerre. 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 LatinTerra 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]
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 −0.0004Ga (billion years) ago.[35] By 4.54±0.04Ga 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 3.8Ga, 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]
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 Eoarcheansedimentary rocks suggests that at least some felsic crust existed as early as 4.4Ga, only 140Ma 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 750Ma, one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia at 600–540Ma, then finally Pangaea, which also began to break apart at 180Ma.[56]
The most recent pattern of ice ages began about 40Ma,[57] and then intensified during the Pleistocene about 3Ma.[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]
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.
During the Neoproterozoic, 1000 to 539Ma, 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, 535Ma, 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 66Ma, 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 66Mys, 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]
Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, about 5–7billion years in the future
Earth's expected long-term future is tied to that of the Sun. Over the next 1.1billion years, solar luminosity will increase by 10%, and over the next 3.5billion years by 40%.[78] Earth's increasing surface temperature will accelerate the inorganic carbon cycle, possibly reducing CO2 concentration to levels lethally low for current plants (10ppm for C4 photosynthesis) in approximately 100–900million years.[79][80] A lack of vegetation would result in the loss of oxygen in the atmosphere, making current animal life impossible.[81] Due to the increased luminosity, Earth's mean temperature may reach 100°C (212°F) in 1.5billion 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 3billion 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 5billion years. Models predict that the Sun will expand to roughly 1AU (150millionkm; 93millionmi), 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 1.7AU (250millionkm; 160millionmi) 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]
To measure the local variation of Earth's topography, geodesy employs an idealized Earth producing a geoid shape. Such a 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 irregular geoid surface, providing a mean sea level (MSL) as a reference level for topographic measurements.[95]
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 510millionkm2 (197millionsqmi).[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.
Earth's land covers 29.2%, or 149millionkm2 (58millionsqmi) 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).[105][106][107] 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 −418m (−1,371ft) at the Dead Sea, to a maximum altitude of 8,848m (29,029ft) at the top of Mount Everest. The mean height of land above sea level is about 797m (2,615ft).[108]
Land can be covered by surface water, snow, ice, artificial structures or vegetation. Most of Earth's land hosts vegetation,[109] but considerable amounts of land are ice sheets (10%,[110] not including the equally large area of land under permafrost)[111] or deserts (33%).[112]
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.[113][114] Earth has an estimated 16.7millionkm2 (6.4millionsqmi) of cropland and 33.5millionkm2 (12.9millionsqmi) of pastureland.[115]
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.[116] Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the mass of the crust.[117]
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 100Ma old. The oldest oceanic crust is located in the Western Pacific and is estimated to be 200Ma old.[123][124] By comparison, the oldest dated continental crust is 4,030Ma,[125] although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to 4,400Ma, 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 55Ma. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75mm/a (3.0in/year)[126] and the Pacific Plate moving 52–69mm/a (2.0–2.7in/year). At the other extreme, the slowest-moving plate is the South American Plate, progressing at a typical rate of 10.6mm/a (0.42in/year).[127]
Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity.[130] The thickness of the crust varies from about 6 kilometres (3.7mi) under the oceans to 30–50km (19–31mi) for the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, which is divided into independently moving tectonic plates.[131]
Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660km (250 and 410mi) below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[132] Earth's inner core may be rotating at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year, although both somewhat higher and much lower rates have also been proposed.[133] The radius of the inner core is about one-fifth of that of Earth. The density increases with depth. Among the Solar System's planetary-sized objects, Earth is the object with the highest density.
Earth's mass is approximately 5.97×1024kg (5.970Yg). It is composed mostly of iron (32.1% by mass), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), 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.[134][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.[135][134]
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.[136] At the center, the temperature may be up to 6,000°C (10,830°F),[137] and the pressure could reach 360GPa (52millionpsi).[138] Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's history, before isotopes with short half-lives were depleted, Earth's heat production was much higher. At approximately 3Gyr, twice the present-day heat would have been produced, increasing the rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today.[139][140]
The mean heat loss from Earth is 87mW m−2, for a global heat loss of 4.42×1013W.[141] A portion of the core's thermal energy is transported toward the crust by mantle plumes, a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[142] More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans.[143]
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 9.8m/s2 (32ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad regional differences in Earth's gravitational field, known as gravity anomalies.[144]
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 3.05×10−5T, with a magnetic dipole moment of 7.79×1022 Am2 at epoch 2000, decreasing nearly 6% per century (although it still remains stronger than its long time average).[145] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes secular variation of the main field and field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[146][147]
The extent of Earth's magnetic field in space defines the magnetosphere. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the day-side of the magnetosphere, to about 10 Earth radii, and extends the night-side magnetosphere into a long tail.[148] Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic bow shock precedes the day-side magnetosphere within the solar wind.[149]Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.[150][151] The ring current is defined by medium-energy particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,[152] and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.[153][154] During magnetic storms and substorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing an aurora.[155]
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[156] Because Earth's solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 ms longer than the mean solar day.[157][158]
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s.[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 8.4ms.[159]
Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.[160][161]
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 150millionkm (93millionmi), which is the basis for the astronomical unit (AU) and is equal to roughly 8.3 light minutes or 380 times Earth's distance to the Moon. Earth orbits the Sun every 365.2564 mean solar days, or one sidereal year. With an apparent movement of the Sun in Earth's sky at a rate of about 1°/day eastward, which is one apparent Sun or Moon diameter every 12hours. Due to this motion, on average it takes 24hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian.
The orbital speed of Earth averages about 29.78km/s (107,200km/h; 66,600mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742km (7,918mi), in seven minutes, and the distance from Earth to the Moon, 384,400km (238,900mi), in about 3.5 hours.[3]
The Moon and Earth orbit a common barycenter every 27.32days 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 29.53days. 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][162]
The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5millionkm (930,000mi) in radius.[163][n 11] This is the maximum distance at which Earth's gravitational influence is stronger than that of the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.[163] Earth, along with the Solar System, is situated in the Milky Way and orbits about 28,000light-years from its center. It is about 20light-years above the galactic plane in the Orion Arm.[164]
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.
During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and the days shorter.[165] Above the Arctic Circle and below the Antarctic Circle there is no daylight at all for part of the year, causing a polar night, and this night extends for several months at the poles themselves. These same latitudes also experience a midnight sun, where the sun remains visible all day.[166][167]
By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.[168]
The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo nutation; a slight, irregular motion with a main period of 18.6years.[169] The orientation (rather than the angle) of Earth's axis also changes over time, precessing around in a complete circle over each 25,800-year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth's equatorial bulge. The poles also migrate a few meters across Earth's surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.[170]
Earth's annual orbit is elliptical rather than circular, and its closest approach to the Sun is called perihelion. In modern times, Earth's perihelion occurs around 3 January, and its aphelion around 4 July. These dates shift over time due to precession and changes to the orbit, the latter of which follows cyclical patterns known as Milankovitch cycles. The annual change in the Earth–Sun distance causes an increase of about 6.8% in solar energy reaching Earth at perihelion relative to aphelion.[171][n 12] Because the Southern Hemisphere is tilted toward the Sun at about the same time that Earth reaches the closest approach to the Sun, the Southern Hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the Southern Hemisphere.[172]
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 planetPluto.[173][174] The natural satellites of other planets are also referred to as "moons", after Earth's.[175] The most widely accepted theory of the Moon's origin, 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] Computer simulations suggest that two blob-like remnants of this protoplanet could be inside the Earth.[176][177]
The gravitational attraction between Earth and the Moon causes lunar tides on Earth.[178] 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.[179] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases.[180] Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38mm/a (1.5in/year). Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23μs/yr—add up to significant changes.[181] During the Ediacaran period, for example, (approximately 620Ma) there were 400±7 days in a year, with each day lasting 21.9±0.4 hours.[182]
The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[183] Some theorists think that without this stabilization against 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.[184][185]
Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[161] This allows total and annular solar eclipses to occur on Earth.[186]
As of September2021[update], there are 4,550 operational, human-made satellites orbiting Earth.[191] 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 (ISS).[192]
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. The mass of the oceans is approximately 1.35×1018metric tons or about 1/4400 of Earth's total mass. The oceans cover an area of 361.8millionkm2 (139.7millionsqmi) with a mean depth of 3,682m (12,080ft), resulting in an estimated volume of 1.332billionkm3 (320millioncumi).[193]
If all of Earth's crustal surface were at the same elevation as a smooth sphere, the depth of the resulting world ocean would be 2.7 to 2.8km (1.68 to 1.74mi).[194] About 97.5% of the water is saline; the remaining 2.5% is fresh water.[195][196] Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.[197] The remaining 30% is ground water, 1% surface water (covering only 2.8% of Earth's land)[198] and other small forms of fresh water deposits such as permafrost, water vapor in the atmosphere, biological binding, etc.[199][200]
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.[201]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.[202]
The average salinity of Earth's oceans is about 35grams of salt per kilogram of seawater (3.5% salt).[203] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[204] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[205] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[206] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.[207]
The abundance of water, particularly liquid water, on Earth's surface is a unique feature that distinguishes it from other planets in the Solar System. Solar System planets with considerable atmospheres do partly host atmospheric water vapor, but they lack surface conditions for stable surface water.[208] 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.[209]
The atmospheric pressure at Earth's sea level averages 101.325kPa (14.696psi),[210] with a scale height of about 8.5km (5.3mi).[3] A dry atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules.[210]Water vapor content varies between 0.01% and 4%[210] but averages about 1%.[3]Clouds cover around two-thirds of Earth's surface, more so over oceans than land.[211] The height of the troposphere varies with latitude, ranging between 8km (5mi) at the poles to 17km (11mi) at the equator, with some variation resulting from weather and seasonal factors.[212]
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7Gya, 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 ultravioletsolar radiation, permitting life on land.[213] Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[214] This last phenomenon is 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),[215] and life on Earth probably would not exist in its current form.[216]
Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.[217] Three-quarters of the atmosphere's mass is contained within the first 11km (6.8mi) of the surface; this lowest layer is called the troposphere.[218] Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy.[219]
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°.[220]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.[221]
Earth receives 1361W/m2 ofsolar irradiance.[222][223] The amount of solar energy that reaches Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4°C (0.7°F) per degree of latitude from the equator.[224] Earth's surface can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.[225]
Further factors that affect a location's climates are its proximity to oceans, the oceanic and atmospheric circulation, and topology.[226] Places close to oceans typically have colder summers and warmer winters, due to the fact that oceans can store large amounts of heat. The wind transports the cold or the heat of the ocean to the land.[227] Atmospheric circulation also plays an important role: San Francisco and Washington DC are both coastal cities at about the same latitude. San Francisco's climate is significantly more moderate as the prevailing wind direction is from sea to land.[228] Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas.[229]
Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and falls to the surface as precipitation.[219] 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.[230]
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 stretching over several hundred kilometers.
The upper atmosphere, the atmosphere above the troposphere,[234] is usually divided into the stratosphere, mesosphere, and thermosphere.[214] 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.[235] Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100km (62mi) above Earth's surface, is a working definition for the boundary between the atmosphere and outer space.[236]
Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth's gravity. This causes a slow but 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.[237] 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.[238] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[239] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[240]
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. Earth's life has been shaping and inhabiting many particular ecosystems on Earth and has eventually expanded globally forming an overarching biosphere.[241]
Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain a metabolism.[245] Plants and other organisms take up nutrients from water, soils and the atmosphere. These nutrients are constantly recycled between different species.[246]
Originating from earlier primates in Eastern Africa 300,000years ago humans have since been migrating and with the advent of agriculture in the 10th millennium BC increasingly settling Earth's land.[252] In the 20th century Antarctica had been the last continent to see a first and until today limited human presence.
Human population has since the 19th century grown exponentially to seven billion in the early 2010s,[253] and is projected to peak at around ten billion in the second half of the 21st century.[254] Most of the growth is expected to take place in sub-Saharan Africa.[254]
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,[255] 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.[256] Furthermore, since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century.[257]
Beyond Earth's surface humans have lived on a temporary basis, with only a few special-purpose deep underground and underwater presences and a few space stations. The human population virtually completely remains on Earth's surface, fully depending on Earth and the environment it sustains. Since the second half of the 20th century, some hundreds of humans have temporarily stayed beyond Earth, a tiny fraction of whom have reached another celestial body, the Moon.[258][259]
Earth has resources that have been exploited by humans.[263] Those termed non-renewable resources, such as fossil fuels, are only replenished over geological timescales.[264] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[265] These deposits are used by humans both for energy production and as feedstock for chemical production.[266] 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.[267] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[268]
Earth's biosphere produces many useful biological products for humans, including food, wood, pharmaceuticals, oxygen, and the recycling of organic waste. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends on dissolved nutrients washed down from the land.[269] In 2019, 39millionkm2 (15millionsqmi) of Earth's land surface consisted of forest and woodlands, 12millionkm2 (4.6millionsqmi) was shrub and grassland, 40millionkm2 (15millionsqmi) were used for animal feed production and grazing, and 11millionkm2 (4.2millionsqmi) were cultivated as croplands.[270] Of the 12–14% of ice-free land that is used for croplands, 2 percentage points were irrigated in 2015.[271] Humans use building materials to construct shelters.[272]
Change in average surface air temperature and drivers for that change. Human activity has caused increased temperatures, with natural forces adding some variability.
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.[250][274] It is estimated that global temperatures in the year 2020 were 1.2°C (2.2°F) warmer than the preindustrial baseline.[275] This increase in temperature, known 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.[251]
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.[276][277] As of 2018, no country meets the basic needs of its population without transgressing planetary boundaries. It is thought possible to provide all basic physical needs globally within sustainable levels of resource use.[278]
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.[290][291] 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.[292]
It was only during the 19th century that geologists realized Earth's age was at least many millions of years.[293]Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400million 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.[294][295]
↑ 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. ISBN978-0-643-09949-4.
↑ Earth's circumference is almost exactly 40,000km 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 LandcoverArchived 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 would feel like tiny imperfections, whereas much of the planet, including the Great Plains and the abyssal plains, would feel smoother.[90]
↑ 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.
↑ 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 .
↑ 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.
↑ As of 4 January 2018, the United States Strategic Command tracked a total of 18,835 artificial objects, mostly debris. See: Anz-Meador, Phillip; Shoots, Debi, eds. (February 2018). "Satellite Box Score"(PDF). Orbital Debris Quarterly News. 22 (1): 12. Archived(PDF) from the original on 2 April 2019. Retrieved 18 April 2018.
Related Research Articles
The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period with its rotation period at 29.5 Earth days, causing the same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the Sun's—are the main drivers of Earth's tides.
Mercury is the first planet from the Sun and the smallest in the Solar System. In English, it is named after the ancient Roman god Mercurius (Mercury), god of commerce and communication, and the messenger of the gods. Mercury is classified as a terrestrial planet, with roughly the same surface gravity as Mars. The surface of Mercury is heavily cratered, as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia, has a diameter of 1,550 km (960 mi), which is about one-third the diameter of the planet. Similarly to the Earth's Moon, Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants.
The Solar System is the gravitationally bound system of the Sun and the objects that orbit it. It formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc. The Sun is a typical star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core, releasing this energy from its outer photosphere. Astronomers classify it as a G-type main-sequence star.
Venus is the second planet from the Sun. It is a terrestrial planet and is the closest in mass and size to its orbital neighbour Earth. Venus has by far the densest atmosphere of the terrestrial planets, composed mostly of carbon dioxide with a thick, global sulfuric acid cloud cover. At the surface it has a mean temperature of 737 K and a pressure 92 times that of Earth's at sea level. These extreme conditions compress carbon dioxide into a supercritical state at Venus's surface.
Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 95 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered independently by Simon Marius and Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus.
Triton is the largest natural satellite of the planet Neptune. It is the only moon of Neptune massive enough to be rounded under its own gravity and hosts a thin but well-structured atmosphere. Triton orbits Neptune in a retrograde orbit—revolving in the opposite direction to the parent planet's rotation—the only large moon in the Solar System to do so. Triton is thought to have once been a dwarf planet from the Kuiper belt, captured into Neptune's orbit by the latter's gravity.
Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about nine times that of Earth. It has an eighth the average density of Earth, but is over 95 times more massive. Even though Saturn is almost as big as Jupiter, Saturn has less than a third the mass of Jupiter. Saturn orbits the Sun at a distance of 9.59 AU (1,434 million km), with an orbital period of 29.45 years.
Titan is the largest moon of Saturn and the second-largest in the Solar System. It is the only moon known to have an atmosphere denser than the Earth's and is the only known object in space—other than Earth—on which there is clear evidence that stable bodies of liquid exist. Titan is one of seven gravitationally rounded moons of Saturn and the second-most distant among them. Frequently described as a planet-like moon, Titan is 50% larger in diameter than Earth's Moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's Ganymede and is larger than Mercury; yet Titan is only 40% as massive as Mercury, because Mercury is mainly iron and rock while much of Titan is ice, which is less dense.
Charon, or (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake, and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).
The Hadean is the first and oldest of the four known geologic eons of Earth's history, starting with the planet's formation about 4.6 billion years ago, and ended 4.031 billion years ago. The interplanetary collision that created the Moon occurred early in this eon. The Hadean eon was succeeded by the Archean eon, with the Late Heavy Bombardment hypothesized to have occurred at the Hadean-Archean boundary.
In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity, such as sexually reproducing, multicellular organisms on Earth, and subsequently human intelligence, required an improbable combination of astrophysical and geological events and circumstances. According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare throughout the universe as a whole. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.
Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from another body, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones (HZ) the only areas in which life might arise.
The origin of water on Earth is the subject of a body of research in the fields of planetary science, astronomy, and astrobiology. Earth is unique among the rocky planets in the Solar System in having oceans of liquid water on its surface. Liquid water, which is necessary for all known forms of life, continues to exist on the surface of Earth because the planet is at a far enough distance from the Sun that it does not lose its water, but not so far that low temperatures cause all water on the planet to freeze.
The exploration of Jupiter has been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, as of 2024, has continued with eight further spacecraft missions in the vicinity of Jupiter and two more en route. All but one of these missions were undertaken by the National Aeronautics and Space Administration (NASA), and all but four were flybys taking detailed observations without landing or entering orbit. These probes make Jupiter the most visited of the Solar System's outer planets as all missions to the outer Solar System have used Jupiter flybys. On 5 July 2016, spacecraft Juno arrived and entered the planet's orbit—the second craft ever to do so. Sending a craft to Jupiter is difficult, mostly due to large fuel requirements and the effects of the planet's harsh radiation environment.
Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life.
Mars is the fourth planet from the Sun. The surface of Mars is orange-red because it is covered in iron(III) oxide dust, giving it the nickname "the Red Planet". Mars is among the brightest objects in Earth's sky, and its high-contrast albedo features have made it a common subject for telescope viewing. It is classified as a terrestrial planet and is the second smallest of the Solar System's planets with a diameter of 6,779 km (4,212 mi). In terms of orbital motion, a Martian solar day (sol) is equal to 24.6 hours, and a Martian solar year is equal to 1.88 Earth years. Mars has two natural satellites that are small and irregular in shape: Phobos and Deimos.
The biological and geological future of Earth can be extrapolated based on the estimated effects of several long-term influences. These include the chemistry at Earth's surface, the cooling rate of the planet's interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun's luminosity. An uncertain factor is the pervasive influence of technology introduced by humans, such as climate engineering, which could cause significant changes to the planet. For example, the current Holocene extinction is being caused by technology, and the effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.
An Earth analog, also called an Earth analogue, Earth twin, or second Earth, is a planet or moon with environmental conditions similar to those found on Earth. The term Earth-like planet is also used, but this term may refer to any terrestrial planet.
Planetary oceanography, also called astro-oceanography or exo-oceanography, is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry, and planetary geology, it only began after the discovery of underground oceans in Saturn's moon Titan and Jupiter's moon Europa. This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even ocean worlds of celestial bodies in the Solar System, from oceans made of liquid carbon with floating diamonds in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.
Planetary habitability in the Solar System is the study that searches the possible existence of past or present extraterrestrial life in those celestial bodies. As exoplanets are too far away and can only be studied by indirect means, the celestial bodies in the Solar System allow for a much more detailed study: direct telescope observation, space probes, rovers and even human spaceflight.
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