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Outer space
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This article is about the particular space between celestial bodies. For the general concept, see Space.
For other uses, see Outer space (disambiguation).
The interface between the Earth's surface and outer space. The Kármán line at an altitude of 100 km (62 mi) is shown. The layers of the atmosphere are drawn to scale, whereas objects within them, such as the International Space Station, are not.
Outer space is the expanse that exists beyond Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2.7 kelvins (−270.45 °C; −454.81 °F).[1] The plasma between galaxies accounts for about half of the baryonic (ordinary) matter in the universe; it has a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins.[2] Local concentrations of matter have condensed into stars and galaxies. Studies indicate that 90% of the mass in most galaxies is in an unknown form, called dark matter, which interacts with other matter through gravitational but not electromagnetic forces.[3][4] Observations suggest that the majority of the mass-energy in the observable universe is dark energy, a type of vacuum energy that is poorly understood.[5][6] Intergalactic space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space.
Outer space does not begin at a definite altitude above the Earth's surface. The Kármán line, an altitude of 100 km (62 mi) above sea level,[7][8] is conventionally used as the start of outer space in space treaties and for aerospace records keeping. The framework for international space law was established by the Outer Space Treaty, which entered into force on 10 October 1967. This treaty precludes any claims of national sovereignty and permits all states to freely explore outer space. Despite the drafting of UN resolutions for the peaceful uses of outer space, anti-satellite weapons have been tested in Earth orbit.
Humans began the physical exploration of space during the 20th century with the advent of high-altitude balloon flights. This was followed by crewed rocket flights and, then, crewed Earth orbit, first achieved by Yuri Gagarin of the Soviet Union in 1961. Due to the high cost of getting into space, human spaceflight has been limited to low Earth orbit and the Moon. On the other hand, uncrewed spacecraft have reached all of the known planets in the Solar System.
Outer space represents a challenging environment for human exploration because of the hazards of vacuum and radiation. Microgravity also has a negative effect on human physiology that causes both muscle atrophy and bone loss. In addition to these health and environmental issues, the economic cost of putting objects, including humans, into space is very high.
Contents
1 Formation and state
2 Environment
3 Effect on biology and human bodies
4 Regions
4.1 Geospace
4.1.1 Cislunar space
4.2 Interplanetary space
4.3 Interstellar space
4.4 Intergalactic space
5 Earth orbit
6 Boundary
7 Legal status
8 Discovery, exploration and applications
8.1 Discovery
8.2 Exploration and application
9 See also
10 References
10.1 Citations
10.2 Sources
11 External links
Formation and state
This is an artist's concept of the metric expansion of space, where a volume of the Universe is represented at each time interval by the circular sections. At left is depicted the rapid inflation from the initial state, followed thereafter by steadier expansion to the present day, shown at right.
Main article: Big Bang
The size of the whole universe is unknown, and it might be infinite in extent.[9] According to the Big Bang theory, the very early Universe was an extremely hot and dense state about 13.8 billion years ago[10] which rapidly expanded. About 380,000 years later the Universe had cooled sufficiently to allow protons and electrons to combine and form hydrogen—the so-called recombination epoch. When this happened, matter and energy became decoupled, allowing photons to travel freely through the continually expanding space.[11] Matter that remained following the initial expansion has since undergone gravitational collapse to create stars, galaxies and other astronomical objects, leaving behind a deep vacuum that forms what is now called outer space.[12] As light has a finite velocity, this theory also constrains the size of the directly observable universe.[11]
The present day shape of the universe has been determined from measurements of the cosmic microwave background using satellites like the Wilkinson Microwave Anisotropy Probe. These observations indicate that the spatial geometry of the observable universe is "flat", meaning that photons on parallel paths at one point remain parallel as they travel through space to the limit of the observable universe, except for local gravity.[13] The flat Universe, combined with the measured mass density of the Universe and the accelerating expansion of the Universe, indicates that space has a non-zero vacuum energy, which is called dark energy.[14]
Estimates put the average energy density of the present day Universe at the equivalent of 5.9 protons per cubic meter, including dark energy, dark matter, and baryonic matter (ordinary matter composed of atoms). The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters.[15] The density of the Universe is clearly not uniform; it ranges from relatively high density in galaxies—including very high density in structures within galaxies, such as planets, stars, and black holes—to conditions in vast voids that have much lower density, at least in terms of visible matter.[16] Unlike matter and dark matter, dark energy seems not to be concentrated in galaxies: although dark energy may account for a majority of the mass-energy in the Universe, dark energy's influence is 5 orders of magnitude smaller than the influence of gravity from matter and dark matter within the Milky Way.[17]
Environment
See also: Planetary habitability
A black background with luminous shapes of various sizes scattered randomly about. They typically have white, red or blue hues.
Part of the Hubble Ultra-Deep Field image showing a typical section of space containing galaxies interspersed by deep vacuum. Given the finite speed of light, this view covers the past 13 billion years of the history of outer space.
Outer space is the closest known approximation to a perfect vacuum. It has effectively no friction, allowing stars, planets, and moons to move freely along their ideal orbits, following the initial formation stage. The deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter.[18] By comparison, the air humans breathe contains about 1025 molecules per cubic meter.[19][20] The low density of matter in outer space means that electromagnetic radiation can travel great distances without being scattered: the mean free path of a photon in intergalactic space is about 1023 km, or 10 billion light years.[21] In spite of this, extinction, which is the absorption and scattering of photons by dust and gas, is an important factor in galactic and intergalactic astronomy.[22]
Stars, planets, and moons retain their atmospheres by gravitational attraction. Atmospheres have no clearly delineated upper boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from outer space.[23] The Earth's atmospheric pressure drops to about 0.032 Pa at 100 kilometres (62 miles) of altitude,[24] compared to 100,000 Pa for the International Union of Pure and Applied Chemistry (IUPAC) definition of standard pressure. Above this altitude, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the Sun and the dynamic pressure of the solar wind. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather.[25]
The temperature of outer space is measured in terms of the kinetic activity of the gas, as it is on Earth. The radiation of outer space has a different temperature than the kinetic temperature of the gas, meaning that the gas and radiation are not in thermodynamic equilibrium.[26][27] All of the observable universe is filled with photons that were created during the Big Bang, which is known as the cosmic microwave background radiation (CMB). (There is quite likely a correspondingly large number of neutrinos called the cosmic neutrino background.[28]) The current black body temperature of the background radiation is about 3 K (−270 °C; −454 °F).[29] The gas temperatures in outer space can vary widely. For example, the temperature in the Boomerang Nebula is 1 K,[30] while the solar corona reaches temperatures over 1.2–2.6 million K.[31]
Magnetic fields have been detected in the space around just about every class of celestial object. Star formation in spiral galaxies can generate small-scale dynamos, creating turbulent magnetic field strengths of around 5–10 μG. The Davis–Greenstein effect causes elongated dust grains to align themselves with a galaxy's magnetic field, resulting in weak optical polarization. This has been used to show ordered magnetic fields exist in several nearby galaxies. Magneto-hydrodynamic processes in active elliptical galaxies produce their characteristic jets and radio lobes. Non-thermal radio sources have been detected even among the most distant, high-z sources, indicating the presence of magnetic fields.[32]
Outside a protective atmosphere and magnetic field, there are few obstacles to the passage through space of energetic subatomic particles known as cosmic rays. These particles have energies ranging from about 106 eV up to an extreme 1020 eV of ultra-high-energy cosmic rays.[33] The peak flux of cosmic rays occurs at energies of about 109 eV, with approximately 87% protons, 12% helium nuclei and 1% heavier nuclei. In the high energy range, the flux of electrons is only about 1% of that of protons.[34] Cosmic rays can damage electronic components and pose a health threat to space travelers.[35] According to astronauts, like Don Pettit, space has a burned/metallic odor that clings to their suits and equipment, similar to the scent of an arc welding torch.[36][37]
Effect on biology and human bodies
See also: Astrobiology, Astrobotany, Plants in space, Animals in space, Effect of spaceflight on the human body, Bioastronautics, and Weightlessness
The lower half shows a blue planet with patchy white clouds. The upper half has a man in a white spacesuit and maneuvering unit against a black background.
Because of the hazards of a vacuum, astronauts must wear a pressurized space suit while off-Earth and outside their spacecraft.
Despite the harsh environment, several life forms have been found that can withstand extreme space conditions for extended periods. Species of lichen carried on the ESA BIOPAN facility survived exposure for ten days in 2007.[38] Seeds of Arabidopsis thaliana and Nicotiana tabacum germinated after being exposed to space for 1.5 years.[39] A strain of bacillus subtilis has survived 559 days when exposed to low-Earth orbit or a simulated martian environment.[40] The lithopanspermia hypothesis suggests that rocks ejected into outer space from life-harboring planets may successfully transport life forms to another habitable world. A conjecture is that just such a scenario occurred early in the history of the Solar System, with potentially microorganism-bearing rocks being exchanged between Venus, Earth, and Mars.[41]
Even at relatively low altitudes in the Earth's atmosphere, conditions are hostile to the human body. The altitude where atmospheric pressure matches the vapor pressure of water at the temperature of the human body is called the Armstrong line, named after American physician Harry G. Armstrong. It is located at an altitude of around 19.14 km (11.89 mi). At or above the Armstrong line, fluids in the throat and lungs boil away. More specifically, exposed bodily liquids such as saliva, tears, and liquids in the lungs boil away. Hence, at this altitude, human survival requires a pressure suit, or a pressurized capsule.[42]
Out in space, sudden exposure of an unprotected human to very low pressure, such as during a rapid decompression, can cause pulmonary barotrauma—a rupture of the lungs, due to the large pressure differential between inside and outside the chest.[citation needed] Even if the subject's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture.[43] Rapid decompression can rupture eardrums and sinuses, bruising and blood seep can occur in soft tissues, and shock can cause an increase in oxygen consumption that leads to hypoxia.[citation needed]
As a consequence of rapid decompression, oxygen dissolved in the blood empties into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrives at the brain, humans lose consciousness after a few seconds and die of hypoxia within minutes.[44] Blood and other body fluids boil when the pressure drops below 6.3 kPa, and this condition is called ebullism.[45] The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid.[46][47] Swelling and ebullism can be reduced by containment in a pressure suit. The Crew Altitude Protection Suit (CAPS), a fitted elastic garment designed in the 1960s for astronauts, prevents ebullism at pressures as low as 2 kPa.[48] Supplemental oxygen is needed at 8 km (5 mi) to provide enough oxygen for breathing and to prevent water loss, while above 20 km (12 mi) pressure suits are essential to prevent ebullism.[49] Most space suits use around 30–39 kPa of pure oxygen, about the same as on the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of nitrogen dissolved in the blood could still cause decompression sickness and gas embolisms if not managed.[50]
Humans evolved for life in Earth gravity, and exposure to weightlessness has been shown to have deleterious effects on human health. Initially, more than 50% of astronauts experience space motion sickness. This can cause nausea and vomiting, vertigo, headaches, lethargy, and overall malaise. The duration of space sickness varies, but it typically lasts for 1–3 days, after which the body adjusts to the new environment. Longer-term exposure to weightlessness results in muscle atrophy and deterioration of the skeleton, or spaceflight osteopenia. These effects can be minimized through a regimen of exercise.[51] Other effects include fluid redistribution, slowing of the cardiovascular system, decreased production of red blood cells, balance disorders, and a weakening of the immune system. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, and puffiness of the face.[52]
During long-duration space travel, radiation can pose an acute health hazard. Exposure to high-energy, ionizing cosmic rays can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the white blood cell count. Over longer durations, symptoms include an increased risk of cancer, plus damage to the eyes, nervous system, lungs and the gastrointestinal tract.[53] On a round-trip Mars mission lasting three years, a large fraction of the cells in an astronaut's body would be traversed and potentially damaged by high energy nuclei.[54] The energy of such particles is significantly diminished by the shielding provided by the walls of a spacecraft and can be further diminished by water containers and other barriers. The impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research is needed to assess the radiation hazards and determine suitable countermeasures.[55]
Regions
Space is a partial vacuum: its different regions are defined by the various atmospheres and "winds" that dominate within them, and extend to the point at which those winds give way to those beyond. Geospace extends from Earth's atmosphere to the outer reaches of Earth's magnetic field, whereupon it gives way to the solar wind of interplanetary space.[56] Interplanetary space extends to the heliopause, whereupon the solar wind gives way to the winds of the interstellar medium.[57] Interstellar space then continues to the edges of the galaxy, where it fades into the intergalactic void.[58]
Geospace
The lower half is the blue-white planet in low illumination. Nebulous red streamers climb upward from the limb of the disk toward the black sky. The Space Shuttle is visible along the left edge.
Aurora australis observed from the Space Shuttle Discovery, on STS-39, May 1991 (orbital altitude: 260 km)
Geospace is the region of outer space near Earth, including the upper atmosphere and magnetosphere.[56] The Van Allen radiation belts lie within the geospace. The outer boundary of geospace is the magnetopause, which forms an interface between the Earth's magnetosphere and the solar wind. The inner boundary is the ionosphere.[59] The variable space-weather conditions of geospace are affected by the behavior of the Sun and the solar wind; the subject of geospace is interlinked with heliophysics—the study of the Sun and its impact on the planets of the Solar System.[60]
The day-side magnetopause is compressed by solar-wind pressure—the subsolar distance from the center of the Earth is typically 10 Earth radii. On the night side, the solar wind stretches the magnetosphere to form a magnetotail that sometimes extends out to more than 100–200 Earth radii.[61][62] For roughly four days of each month, the lunar surface is shielded from the solar wind as the Moon passes through the magnetotail.[63]
Geospace is populated by electrically charged particles at very low densities, the motions of which are controlled by the Earth's magnetic field. These plasmas form a medium from which storm-like disturbances powered by the solar wind can drive electrical currents into the Earth's upper atmosphere. Geomagnetic storms can disturb two regions of geospace, the radiation belts and the ionosphere. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, interfering with shortwave radio communication and GPS location and timing.[64] Magnetic storms can also be a hazard to astronauts, even in low Earth orbit. They also create aurorae seen at high latitudes in an oval surrounding the geomagnetic poles.[65]
Although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant drag on satellites.[66] This region contains material left over from previous crewed and uncrewed launches that are a potential hazard to spacecraft. Some of this debris re-enters Earth's atmosphere periodically.[67]
Cislunar space
Lunar Gateway, one of the planned space stations for crewed cislunar travel in the 2020s
Earth's gravity keeps the Moon in orbit at an average distance of 384,403 km (238,857 mi). The region outside Earth's atmosphere and extending out to just beyond the Moon's orbit, including the Lagrange points, is sometimes referred to as cislunar space.[68]
Deep space is defined by the United States government and others as any region beyond cislunar space.[69][70][71][72] The International Telecommunication Union responsible for radio communication (including satellites) defines the beginning of deep space at about 5 times that distance (2×106 km).[73]
The region where Earth's gravity remains dominant against gravitational perturbations from the Sun is called the Hill sphere.[74] This extends into translunar space to a distance of roughly 1% of the mean distance from Earth to the Sun,[75] or 1.5 million km (0.93 million mi).
Interplanetary space
Main article: Interplanetary medium
At lower left, a white coma stands out against a black background. Nebulous material streams away to the top and left, slowly fading with distance.
The sparse plasma (blue) and dust (white) in the tail of comet Hale–Bopp are being shaped by pressure from solar radiation and the solar wind, respectively
Interplanetary space is defined by the solar wind, a continuous stream of charged particles emanating from the Sun that creates a very tenuous atmosphere (the heliosphere) for billions of kilometers into space. This wind has a particle density of 5–10 protons/cm3 and is moving at a velocity of 350–400 km/s (780,000–890,000 mph).[76] Interplanetary space extends out to the heliopause where the influence of the galactic environment starts to dominate over the magnetic field and particle flux from the Sun.[57] The distance and strength of the heliopause varies depending on the activity level of the solar wind.[77] The heliopause in turn deflects away low-energy galactic cosmic rays, with this modulation effect peaking during solar maximum.[78]
The volume of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth. This space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust,[79] small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy.[80] A cloud of interplanetary dust is visible at night as a faint band called the zodiacal light.[81]
Interplanetary space contains the magnetic field generated by the Sun.[76] There are also magnetospheres generated by planets such as Jupiter, Saturn, Mercury and the Earth that have their own magnetic fields. These are shaped by the influence of the solar wind into the approximation of a teardrop shape, with the long tail extending outward behind the planet. These magnetic fields can trap particles from the solar wind and other sources, creating belts of charged particles such as the Van Allen radiation belts. Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind.[82]
Interstellar space
Main article: Interstellar medium
"Interstellar space" redirects here. For the album, see Interstellar Space.
Patchy orange and blue nebulosity against a black background, with a curved orange arc wrapping around a star at the center.
Bow shock formed by the magnetosphere of the young star LL Orionis (center) as it collides with the Orion Nebula flow
Interstellar space is the physical space within a galaxy beyond the influence each star has upon the encompassed plasma.[58] The contents of interstellar space are called the interstellar medium. Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms. This is enriched with trace amounts of heavier atoms formed through stellar nucleosynthesis. These atoms are ejected into the interstellar medium by stellar winds or when evolved stars begin to shed their outer envelopes such as during the formation of a planetary nebula.[83] The cataclysmic explosion of a supernova generates an expanding shock wave consisting of ejected materials that further enrich the medium.[84] The density of matter in the interstellar medium can vary considerably: the average is around 106 particles per m3,[85] but cold molecular clouds can hold 108–1012 per m3.[26][83]
A number of molecules exist in interstellar space, as can tiny 0.1 μm dust particles.[86] The tally of molecules discovered through radio astronomy is steadily increasing at the rate of about four new species per year. Large regions of higher density matter known as molecular clouds allow chemical reactions to occur, including the formation of organic polyatomic species. Much of this chemistry is driven by collisions. Energetic cosmic rays penetrate the cold, dense clouds and ionize hydrogen and helium, resulting, for example, in the trihydrogen cation. An ionized helium atom can then split relatively abundant carbon monoxide to produce ionized carbon, which in turn can lead to organic chemical reactions.[87]
The local interstellar medium is a region of space within 100 parsecs (pc) of the Sun, which is of interest both for its proximity and for its interaction with the Solar System. This volume nearly coincides with a region of space known as the Local Bubble, which is characterized by a lack of dense, cold clouds. It forms a cavity in the Orion Arm of the Milky Way galaxy, with dense molecular clouds lying along the borders, such as those in the constellations of Ophiuchus and Taurus. (The actual distance to the border of this cavity varies from 60 to 250 pc or more.) This volume contains about 104–105 stars and the local interstellar gas counterbalances the astrospheres that surround these stars, with the volume of each sphere varying depending on the local density of the interstellar medium. The Local Bubble contains dozens of warm interstellar clouds with temperatures of up to 7,000 K and radii of 0.5–5 pc.[88]
When stars are moving at sufficiently high peculiar velocities, their astrospheres can generate bow shocks as they collide with the interstellar medium. For decades it was assumed that the Sun had a bow shock. In 2012, data from Interstellar Boundary Explorer (IBEX) and NASA's Voyager probes showed that the Sun's bow shock does not exist. Instead, these authors argue that a subsonic bow wave defines the transition from the solar wind flow to the interstellar medium.[89][90] A bow shock is the third boundary of an astrosphere after the termination shock and the astropause (called the heliopause in the Solar System).[90]
Intergalactic space
Structure of the Universe
Matter distribution in a cubic section of the universe. The blue fiber structures represent the matter and the empty regions in between represent the cosmic voids of the intergalactic medium.
A star-forming region in the Large Magellanic Cloud, perhaps the closest Galaxy to Earth's Milky Way
Main articles: Warm–hot intergalactic medium, Intracluster medium, and Intergalactic dust
Intergalactic space is the physical space between galaxies. Studies of the large scale distribution of galaxies show that the Universe has a foam-like structure, with groups and clusters of galaxies lying along filaments that occupy about a tenth of the total space. The remainder forms huge voids that are mostly empty of galaxies. Typically, a void spans a distance of (10–40) h−1 Mpc, where h is the Hubble constant in units of 100 km s−1 Mpc−1, or the dimensionless Hubble constant.[91]
Surrounding and stretching between galaxies, there is a rarefied plasma[92] that is organized in a galactic filamentary structure.[93] This material is called the intergalactic medium (IGM). The density of the IGM is 5–200 times the average density of the Universe.[94] It consists mostly of ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 105 K to 107 K,[2] which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). (Although the plasma is very hot by terrestrial standards, 105 K is often called "warm" in astrophysics.) Computer simulations and observations indicate that up to half of the atomic matter in the Universe might exist in this warm–hot, rarefied state.[94][95][96] When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 108 K and above in the so-called intracluster medium (ICM).[97]
Earth orbit
Main article: Geocentric orbit
A spacecraft enters orbit when its centripetal acceleration due to gravity is less than or equal to the centrifugal acceleration due to the horizontal component of its velocity. For a low Earth orbit, this velocity is about 7,800 m/s (28,100 km/h; 17,400 mph);[98] by contrast, the fastest piloted airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2,200 m/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15.[99]
To achieve an orbit, a spacecraft must travel faster than a sub-orbital spaceflight. The energy required to reach Earth orbital velocity at an altitude of 600 km (370 mi) is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude.[100] Spacecraft with a perigee below about 2,000 km (1,200 mi) are subject to drag from the Earth's atmosphere,[101] which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere. Below about 300 km (190 mi), decay becomes more rapid with lifetimes measured in days. Once a satellite descends to 180 km (110 mi), it has only hours before it vaporizes in the atmosphere.[66] The escape velocity required to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11,200 m/s (40,300 km/h; 25,100 mph).[102]
Boundary
For the boundary of the universe, see observable universe.
A white rocketship with oddly-shaped wings at rest on a runway.
SpaceShipOne completed the first human private spaceflight in 2004, reaching an altitude of 100.12 km (62.21 mi).[103]
There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases. There are several standard boundary designations, namely:
The Fédération Aéronautique Internationale has established the Kármán line at an altitude of 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about 100 km (62 mi), as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself.[7][8]
The United States designates people who travel above an altitude of 50 mi (80 km) as astronauts.[104]
NASA's Space Shuttle used 400,000 feet (122 km, 76 mi) as its re-entry altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag becomes noticeable, thus beginning the process of switching from steering with thrusters to maneuvering with aerodynamic control surfaces.[105]
In 2009, scientists reported detailed measurements with a Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at 118 km (73.3 mi) above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over 268 m/s (600 mph).[106][107]
Legal status
Main article: Space law
See also: Bogota Declaration
At top, a dark rocket is emitting a bright plume of flame against a blue sky. Underneath, a column of smoke is partly concealing a navy ship.
2008 launch of the SM-3 missile used to destroy American reconnaissance satellite USA-193
The Outer Space Treaty provides the basic framework for international space law. It covers the legal use of outer space by nation states, and includes in its definition of outer space the Moon and other celestial bodies. The treaty states that outer space is free for all nation states to explore and is not subject to claims of national sovereignty, calling outer space the "province of all mankind". This status as a common heritage of mankind has been used, though not without opposition, to enforce the right to access and shared use of outer space for all nations equally, particularly non-spacefaring nations.[108] It also prohibits the development of nuclear weapons in outer space. The treaty was passed by the United Nations General Assembly in 1963 and signed in 1967 by the USSR, the United States of America and the United Kingdom. As of 2017, 105 state parties have either ratified or acceded to the treaty. An additional 25 states signed the treaty, without ratifying it.[109][110]
Since 1958, outer space has been the subject of multiple United Nations resolutions. Of these, more than 50 have been concerning the international co-operation in the peaceful uses of outer space and preventing an arms race in space.[111] Four additional space law treaties have been negotiated and drafted by the UN's Committee on the Peaceful Uses of Outer Space. Still, there remains no legal prohibition against deploying conventional weapons in space, and anti-satellite weapons have been successfully tested by the US, USSR, China,[112] and in 2019, India.[113] The 1979 Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. The treaty has not been ratified by any nation that currently practices human spaceflight.[114]
In 1976, eight equatorial states (Ecuador, Colombia, Brazil, Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia. With their "Declaration of the First Meeting of Equatorial Countries", or "the Bogotá Declaration", they claimed control of the segment of the geosynchronous orbital path corresponding to each country.[115] These claims are not internationally accepted.[116]
Discovery, exploration and applications
See also: Space science
Discovery
In 350 BCE, Greek philosopher Aristotle suggested that nature abhors a vacuum, a principle that became known as the horror vacui. This concept built upon a 5th-century BCE ontological argument by the Greek philosopher Parmenides, who denied the possible existence of a void in space.[117] Based on this idea that a vacuum could not exist, in the West it was widely held for many centuries that space could not be empty.[118] As late as the 17th century, the French philosopher René Descartes argued that the entirety of space must be filled.[119]
In ancient China, the 2nd-century astronomer Zhang Heng became convinced that space must be infinite, extending well beyond the mechanism that supported the Sun and the stars. The surviving books of the Hsüan Yeh school said that the heavens were boundless, "empty and void of substance". Likewise, the "sun, moon, and the company of stars float in the empty space, moving or standing still".[120]
The Italian scientist Galileo Galilei knew that air had mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. It would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a partial vacuum in 1643. This experiment resulted in the first mercury barometer and created a scientific sensation in Europe. The French mathematician Blaise Pascal reasoned that if the column of mercury was supported by air, then the column ought to be shorter at higher altitude where the air pressure is lower.[121] In 1648, his brother-in-law, Florin Périer, repeated the experiment on the Puy de Dôme mountain in central France and found that the column was shorter by three inches. This decrease in pressure was further demonstrated by carrying a half-full balloon up a mountain and watching it gradually expand, then contract upon descent.[122]
A glass display case holds a mechanical device with a lever arm, plus two metal hemispheres attached to draw ropes
The original Magdeburg hemispheres (lower left) used to demonstrate Otto von Guericke's vacuum pump (right)
In 1650, German scientist Otto von Guericke constructed the first vacuum pump: a device that would further refute the principle of horror vacui. He correctly noted that the atmosphere of the Earth surrounds the planet like a shell, with the density gradually declining with altitude. He concluded that there must be a vacuum between the Earth and the Moon.[123]
Back in the 15th century, German theologian Nicolaus Cusanus speculated that the Universe lacked a center and a circumference. He believed that the Universe, while not infinite, could not be held as finite as it lacked any bounds within which it could be contained.[124] These ideas led to speculations as to the infinite dimension of space by the Italian philosopher Giordano Bruno in the 16th century. He extended the Copernican heliocentric cosmology to the concept of an infinite Universe filled with a substance he called aether, which did not resist the motion of heavenly bodies.[125] English philosopher William Gilbert arrived at a similar conclusion, arguing that the stars are visible to us only because they are surrounded by a thin aether or a void.[126] This concept of an aether originated with ancient Greek philosophers, including Aristotle, who conceived of it as the medium through which the heavenly bodies move.[127]
The concept of a Universe filled with a luminiferous aether retained support among some scientists until the early 20th century. This form of aether was viewed as the medium through which light could propagate.[128] In 1887, the Michelson–Morley experiment tried to detect the Earth's motion through this medium by looking for changes in the speed of light depending on the direction of the planet's motion. The null result indicated something was wrong with the concept. The idea of the luminiferous aether was then abandoned. It was replaced by Albert Einstein's theory of special relativity, which holds that the speed of light in a vacuum is a fixed constant, independent of the observer's motion or frame of reference.[129][130]
The first professional astronomer to support the concept of an infinite Universe was the Englishman Thomas Digges in 1576.[131] But the scale of the Universe remained unknown until the first successful measurement of the distance to a nearby star in 1838 by the German astronomer Friedrich Bessel. He showed that the star system 61 Cygni had a parallax of just 0.31 arcseconds (compared to the modern value of 0.287″). This corresponds to a distance of over 10 light years.[132] In 1917, Heber Curtis noted that novae in spiral nebulae were, on average, 10 magnitudes fainter than galactic novae, suggesting that the former are 100 times further away.[133] The distance to the Andromeda Galaxy was determined in 1923 by American astronomer Edwin Hubble by measuring the brightness of cepheid variables in that galaxy, a new technique discovered by Henrietta Leavitt.[134] This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the Milky Way.[135]
The modern concept of outer space is based on the "Big Bang" cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître.[136] This theory holds that the universe originated from a very dense form that has since undergone continuous expansion.
The earliest known estimate of the temperature of outer space was by the Swiss physicist Charles É. Guillaume in 1896. Using the estimated radiation of the background stars, he concluded that space must be heated to a temperature of 5–6 K. British physicist Arthur Eddington made a similar calculation to derive a temperature of 3.18 K in 1926. German physicist Erich Regener used the total measured energy of cosmic rays to estimate an intergalactic temperature of 2.8 K in 1933.[137] American physicists Ralph Alpher and Robert Herman predicted 5 K for the temperature of space in 1948, based on the gradual decrease in background energy following the then-new Big Bang theory.[137] The modern measurement of the cosmic microwave background is about 2.7K.
The term outward space was used in 1842 by the English poet Lady Emmeline Stuart-Wortley in her poem "The Maiden of Moscow".[138] The expression outer space was used as an astronomical term by Alexander von Humboldt in 1845.[139] It was later popularized in the writings of H. G. Wells in 1901.[140] The shorter term space is older, first used to mean the region beyond Earth's sky in John Milton's Paradise Lost in 1667.[141][142]
Exploration and application
Main articles: Space exploration and Human presence in space
See also: Astronautics, Spaceflight, Benefits of space exploration, Earth observation, Commercialization of space, Human spaceflight, and Space habitation
The first image taken by a human of the whole Earth, probably photographed by William Anders of Apollo 8.[143] South is up; South America is in the middle.
For most of human history, space was explored by observations made from the Earth's surface—initially with the unaided eye and then with the telescope. Before reliable rocket technology, the closest that humans had come to reaching outer space was through balloon flights. In 1935, the U.S. Explorer II crewed balloon flight reached an altitude of 22 km (14 mi).[144] This was greatly exceeded in 1942 when the third launch of the German A-4 rocket climbed to an altitude of about 80 km (50 mi). In 1957, the uncrewed satellite Sputnik 1 was launched by a Russian R-7 rocket, achieving Earth orbit at an altitude of 215–939 kilometres (134–583 mi).[145] This was followed by the first human spaceflight in 1961, when Yuri Gagarin was sent into orbit on Vostok 1. The first humans to escape low-Earth orbit were Frank Borman, Jim Lovell and William Anders in 1968 on board the U.S. Apollo 8, which achieved lunar orbit[146] and reached a maximum distance of 377,349 km (234,474 mi) from the Earth.[147]
The first spacecraft to reach escape velocity was the Soviet Luna 1, which performed a fly-by of the Moon in 1959.[148] In 1961, Venera 1 became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of Venus, although contact was lost before reaching Venus. The first successful planetary mission was the 1962 fly-by of Venus by Mariner 2.[149] The first fly-by of Mars was by Mariner 4 in 1964. Since that time, uncrewed spacecraft have successfully examined each of the Solar System's planets, as well their moons and many minor planets and comets. They remain a fundamental tool for the exploration of outer space, as well as for observation of the Earth.[150] In August 2012, Voyager 1 became the first man-made object to leave the Solar System and enter interstellar space.[151]
The absence of air makes outer space an ideal location for astronomy at all wavelengths of the electromagnetic spectrum. This is evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing light from more than 13 billion years ago—almost to the time of the Big Bang—to be observed.[152] Not every location in space is ideal for a telescope. The interplanetary zodiacal dust emits a diffuse near-infrared radiation that can mask the emission of faint sources such as extrasolar planets. Moving an infrared telescope out past the dust increases its effectiveness.[153] Likewise, a site like the Daedalus crater on the far side of the Moon could shield a radio telescope from the radio frequency interference that hampers Earth-based observations.[154]
Uncrewed spacecraft in Earth orbit are an essential technology of modern civilization. They allow direct monitoring of weather conditions, relay long-range communications like television, provide a means of precise navigation, and allow remote sensing of the Earth. The latter role serves a wide variety of purposes, including tracking soil moisture for agriculture, prediction of water outflow from seasonal snow packs, detection of diseases in plants and trees, and surveillance of military activities.[155]
The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those requiring ultraclean surfaces.[156] Like asteroid mining, space manufacturing would require a large financial investment with little prospect of immediate return.[157] An important factor in the total expense is the high cost of placing mass into Earth orbit: $8,000–$26,000 per kg, according to a 2006 estimate (allowing for inflation since then).[158] The cost of access to space has declined since 2013. Partially reusable rockets such as the Falcon 9 have lowered access to space below 3500 dollars per kilogram. With these new rockets the cost to send materials into space remains prohibitively high for many industries. Proposed concepts for addressing this issue include, fully reusable launch systems, non-rocket spacelaunch, momentum exchange tethers, and space elevators.[159]
Interstellar travel for a human crew remains at present only a theoretical possibility. The distances to the nearest stars mean it would require new technological developments and the ability to safely sustain crews for journeys lasting several decades. For example, the Daedalus Project study, which proposed a spacecraft powered by the fusion of deuterium and helium-3, would require 36 years to reach the "nearby" Alpha Centauri system. Other proposed interstellar propulsion systems include light sails, ramjets, and beam-powered propulsion. More advanced propulsion systems could use antimatter as a fuel, potentially reaching relativistic velocities.[160]
See also
Outer space portal
Spaceflight portal
Astronomy portal
Earth's location in the Universe
List of government space agencies
List of topics in space
Outline of space science
Panspermia
Space and survival
Space environment
Space race
Space station
Space technology
Space weather
Space weathering
Timeline of knowledge about the interstellar and intergalactic medium
Timeline of Solar System exploration
Timeline of spaceflight
Laika, first living creature in space
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Earth
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This article is about the planet. For its human aspects, see World. For other uses, see Earth (disambiguation) and Planet Earth (disambiguation).
Earth Astronomical symbol of Earth
The Blue Marble photograph of Earth, taken by the Apollo 17 mission. The Arabian peninsula, Africa and Madagascar lie in the upper half of the disc, whereas Antarctica is at the bottom.
The Blue Marble, the most widely used photograph of Earth,[1][2] taken by the Apollo 17 mission in 1972
Designations
Alternative names Gaia, Terra, Tellus, the world, the globe
Adjectives Earthly, terrestrial, terran, tellurian
Orbital characteristics
Epoch J2000[n 1]
Aphelion 152100000 km (94500000 mi)[n 2]
Perihelion 147095000 km (91401000 mi)[n 2]
Semi-major axis 149598023 km (92955902 mi)[3]
Eccentricity 0.0167086[3]
Orbital period 365.256363004 d[4]
(31558.1497635 ks)
Average orbital speed 29.78 km/s[5]
(107200 km/h; 66600 mph)
Mean anomaly 358.617°
Inclination
7.155° to the Sun's equator;
1.57869°[6] to invariable plane;
0.00005° to J2000 ecliptic
Longitude of ascending node −11.26064°[5] to J2000 ecliptic
Time of perihelion 2021-Jan-02 13:59[7]
Argument of perihelion 114.20783°[5]
Satellites
1 natural satellite: the Moon
5 quasi-satellites
>3 300 operational artificial satellites[8]
>18 000 tracked space debris[n 3]
Physical characteristics
Mean radius 6371.0 km (3958.8 mi)[9]
Equatorial radius 6378.137 km (3963.191 mi)[10][11]
Polar radius 6356.752 km (3949.903 mi)[12]
Flattening 1/298.257222101 (ETRS89)[13]
Circumference
40075.017 km equatorial (24901.461 mi)[11]
40007.86 km meridional (24859.73 mi)[14][n 4]
Surface area
510067420 km2 (196938130 sq mi)[15][n 5]
148940000 km2 land (57510000 sq mi)
361132000 km2 water (139434000 sq mi)
Volume 1.08321×1012 km3 (2.59876×1011 cu mi)[5]
Mass 5.97237×1024 kg (1.31668×1025 lb)[16]
(3.0×10−6 M☉)
Mean density 5.514 g/cm3 (0.1992 lb/cu in)[5]
Surface gravity 9.80665 m/s2 (1 g; 32.1740 ft/s2)[17]
Moment of inertia factor 0.3307[18]
Escape velocity 11.186 km/s[5] (40270 km/h; 25020 mph)
Rotation period 1.0 d
(24h 00m 00s) average synodic rotation period (solar day)
Sidereal rotation period 0.99726968 d[19]
(23h 56m 4.100s)
Equatorial rotation velocity 0.4651 km/s[20]
(1674.4 km/h; 1040.4 mph)
Axial tilt 23.4392811°[4]
Albedo
0.367 geometric[5]
0.306 Bond[5]
Surface temp. min mean max
Celsius −89.2 °C[21] 14 °C (1961–90)[22] 56.7 °C[23]
Fahrenheit −128.5 °F 57.2 °F (1961–90) 134.0 °F
Atmosphere
Surface pressure 101.325 kPa (at MSL)
Composition by volume
78.08% nitrogen (N
2; dry air)[5]
20.95% oxygen (O
2)
~ 1% water vapor (climate variable)
0.9340% argon
0.0413% carbon dioxide[24]
0.00182% neon[5]
0.00052% helium
0.00019% methane
0.00011% krypton
0.00006% hydrogen
Earth is the third planet from the Sun and the only astronomical object known to harbor and support life. About 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is covered with water, mostly by oceans, seas, gulfs, and other salt-water bodies, but also by lakes, rivers, and other freshwater, which together constitute the hydrosphere. Much of Earth's polar regions are covered in ice. Earth's outer layer is divided into several rigid tectonic plates that migrate across the surface over many millions of years, while its interior remains active with a solid iron inner core, a liquid outer core that generates Earth's magnetic field, and a convective mantle that drives plate tectonics.
Earth's atmosphere consists mostly of nitrogen and oxygen. More solar energy is received by tropical regions than polar regions and is redistributed by atmospheric and ocean circulation. Greenhouse gases also play an important role in regulating the surface temperature. A region's climate is not only determined by latitude, but also by elevation and proximity to moderating oceans, among other factors. Severe weather, such as tropical cyclones, thunderstorms, and heatwaves, occurs in most areas and greatly impacts life.
Earth's gravity interacts with other objects in space, especially the Moon, which is Earth's only natural satellite. Earth orbits around the Sun in about 365.25 days. Earth's axis of rotation is tilted with respect to its orbital plane, producing seasons on Earth. The gravitational interaction between Earth and the Moon causes tides, stabilizes Earth's orientation on its axis, and gradually slows its rotation. Earth is the densest planet in the Solar System and the largest and most massive of the four rocky planets.
According to radiometric dating estimation and other evidence, Earth formed over 4.5 billion years ago. Within the first billion years of Earth's history, life appeared in the oceans and began to affect Earth's atmosphere and surface, leading to the proliferation of anaerobic and, later, aerobic organisms. Some geological evidence indicates that life may have arisen as early as 4.1 billion years ago. Since then, the combination of Earth's distance from the Sun, physical properties, and geological history have allowed life to evolve and thrive. In the history of life on Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinctions. Over 99% of all species that ever lived on Earth are extinct. Almost 8 billion humans live on Earth and depend on its biosphere and natural resources for their survival. Humans increasingly impact Earth's surface, hydrology, atmospheric processes, and other life.
Contents
1 Etymology
2 Chronology
2.1 Formation
2.2 Geological history
2.3 Origin of life and evolution
2.4 Future
3 Physical characteristics
3.1 Size and shape
3.2 Chemical composition
3.3 Internal structure
3.4 Heat
3.5 Tectonic plates
3.6 Surface
3.7 Gravitational field
3.8 Magnetic field
4 Orbit and rotation
4.1 Rotation
4.2 Orbit
4.3 Axial tilt and seasons
5 Earth-Moon system
5.1 Moon
5.2 Asteroids and artificial satellites
6 Hydrosphere
7 Atmosphere
7.1 Weather and climate
7.2 Upper atmosphere
8 Life on Earth
9 Human geography
9.1 Natural resources and land use
9.2 Humans and climate
10 Cultural and historical viewpoint
11 See also
12 Notes
13 References
14 External links
Etymology
The modern English word Earth developed, via Middle English, from an Old English noun most often spelled eorðe.[25] It has cognates in every Germanic language, and their ancestral root has been reconstructed as *erþō. In its earliest attestation, the word eorðe was already being used to translate the many senses of Latin terra and Greek γῆ gē: the ground, its soil, dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Roman Terra/Tellūs and Greek Gaia, Earth may have been a personified goddess in Germanic paganism: late Norse mythology included Jörð ('Earth'), a giantess often given as the mother of Thor.[26]
Historically, earth has been written in lowercase. From early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, and the earth was also written the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets, though earth and forms with the remain common.[25] House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (for example, "Earth's atmosphere") but writes it in lowercase when preceded by the (for example, "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[27]
Occasionally, the name Terra /ˈtɛrə/ is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others,[28] while in poetry Tellus /ˈtɛləs/ has been used to denote personification of the Earth.[29] Terra is also the name of the planet in some Romance languages (languages that evolved from Latin) like Italian and Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings (like the Spanish Tierra and the French Terre). The Latinate form Gæa or Gaea (English: /ˈdʒiːə/) 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 /ˈɡaɪə/ rather than the more classical English /ˈɡeɪə/.[30]
There are a number of adjectives for the planet Earth. From Earth itself comes earthly. From the Latin Terra comes terran /ˈtɛrən/,[31] terrestrial /təˈrɛstriəl/,[32] and (via French) terrene /təˈriːn/,[33] and from the Latin Tellus comes tellurian /tɛˈlʊəriən/[34] and telluric.[35]
Chronology
Main article: History of Earth
Formation
Artist's impression of the early Solar System's planetary disk
The oldest material found in the Solar System is dated to 4.5682+0.0002
−0.0004 Ga (billion years) ago.[36] By 4.54±0.04 Ga the primordial Earth had formed.[37] 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.[38]
Estimates of the age of the Moon range from 4.5 Ga to significantly younger.[39] 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.[40] It hit Earth with a glancing blow and some of its mass merged with Earth.[41][42] Between approximately 4.1 and 3.8 Ga, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth.[43]
Geological history
Main article: Geological history of Earth
Carboniferous rocks that were folded, uplifted and eroded during the orogeny that completed the formation of the Pangaea supercontinent, before deposition of the overlying Triassic strata, in the Algarve Basin, which marked the start of its break-up
Earth's atmosphere and oceans were formed by volcanic activity and outgassing.[44] Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets.[45] Sufficient water to fill the oceans may have been on Earth since it formed.[46] In this model, atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity.[47] By 3.5 Ga, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[48]
As the molten outer layer of Earth cooled it formed the first solid crust, which is thought to have been mafic in composition. The first continental crust, which was more felsic in composition, formed by the partial melting of this mafic crust. The presence of grains of the mineral zircon of Hadean age in Eoarchean sedimentary rocks suggests that at least some felsic crust existed as early as 4.4 Ga, only 140 Ma after Earth's formation.[49] There are two main models of how this initial small volume of continental crust evolved to reach its current abundance:[50] (1) a relatively steady growth up to the present day,[51] 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,[52][53] 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.[54]
New continental crust forms as a result of plate tectonics, a process ultimately driven by the continuous loss of heat from Earth's interior. Over the period of hundreds of millions of years, tectonic forces have caused areas of continental crust to group together to form supercontinents that have subsequently broken apart. At approximately 750 Ma, one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia at 600–540 Ma, then finally Pangaea, which also began to break apart at 180 Ma.[55]
The most recent pattern of ice ages began about 40 Ma,[56] and then intensified during the Pleistocene about 3 Ma.[57] 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.[58] The Last Glacial Period, colloquially called the "last ice age", covered large parts of the continents, up to the middle latitudes, in ice and ended about 11,700 years ago.[59]
Origin of life and evolution
Life timeline
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Water
Single-celled life
Photosynthesis
Eukaryotes
Multicellular life
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Mammals
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Earth formed
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Earliest water
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Earliest life
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LHB meteorites
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Earliest oxygen
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Pongola glaciation*
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Atmospheric oxygen
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Huronian glaciation*
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Earliest multicellular life
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Earliest fungi
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Sexual reproduction
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Earliest plants
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Earliest animals
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Cryogenian ice age*
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Ediacaran biota
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Cambrian explosion
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Andean glaciation*
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Earliest tetrapods
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Karoo ice age*
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Earliest apes / humans
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Quaternary ice age*
(million years ago)*Ice Ages
Main articles: Origin of Life and Evolutionary history of life
Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose.[60] The evolution of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant molecular oxygen (O
2) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer (O
3) in the upper atmosphere.[61] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[62] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized Earth's surface.[63] Among the earliest fossil evidence for life is microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia,[64] biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland,[65] and remains of biotic material found in 4.1 billion-year-old rocks in Western Australia.[66][67] The earliest direct evidence of life on Earth is contained in 3.45 billion-year-old Australian rocks showing fossils of microorganisms.[68][69]
During the Neoproterozoic, 1000 to 541 Ma, much of Earth might have been covered in ice. This hypothesis has been termed "Snowball Earth", and it is of particular interest because it preceded the Cambrian explosion, when multicellular life forms significantly increased in complexity.[70][71] Following the Cambrian explosion, 535 Ma, there have been at least five major mass extinctions and many minor ones.[72][73] Apart from the proposed current Holocene extinction event, the most recent was 66 Ma, when an asteroid impact triggered the extinction of the non-avian dinosaurs and other large reptiles, but largely spared small animals such as insects, mammals, lizards and birds. Mammalian life has diversified over the past 66 Mys, and several million years ago an African ape gained the ability to stand upright.[74] 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.[75]
Future
Main article: Future of Earth
See also: Global catastrophic risk
Because carbon dioxide (CO
2) has a long lifespan in the atmosphere, moderate human CO
2 emissions may postpone the next glacial inception by 100,000 years.[76] Earth's expected long-term future is tied to that of the Sun. Over the next 1.1 billion years, solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%.[77] Earth's increasing surface temperature will accelerate the inorganic carbon cycle, reducing CO
2 concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 100–900 million years.[78][79] The lack of vegetation will result in the loss of oxygen in the atmosphere, making animal life impossible.[80] Due to the increased luminosity, Earth's mean temperature may reach 100 °C (212 °F) in 1.5 billion years, and all ocean water will evaporate and be lost to space within an estimated 1.6 to 3 billion years.[81] 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.[81][82]
The Sun will evolve to become a red giant in about 5 billion years. Models predict that the Sun will expand to roughly 1 AU (150 million km; 93 million mi), about 250 times its present radius.[77][83] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU (250 million km; 160 million mi) from the Sun when the star reaches its maximum radius.[77]
Physical characteristics
Size and shape
Main article: Figure of the Earth
Further information: Earth radius and Earth's circumference
See also: List of highest mountains on Earth
Chimborazo, whose summit is the point on Earth's surface that is farthest from Earth's center[84]
The shape of Earth is nearly spherical. There is a small flattening at the poles and bulging around the equator due to Earth's rotation.[85] Therefore, a better approximation of Earth's shape is an oblate spheroid, whose equatorial diameter is 43 kilometres (27 mi) larger than the pole-to-pole diameter.[86]
The point on the surface farthest from Earth's center of mass is the summit of the equatorial Chimborazo volcano in Ecuador (6,384.4 km or 3,967.1 mi).[87][88][89] The average diameter of the reference spheroid is 12,742 kilometres (7,918 mi). Local topography deviates from this idealized spheroid, although on a global scale these deviations are small compared to Earth's radius: the maximum deviation of only 0.17% is at the Mariana Trench (10,925 metres or 35,843 feet below local sea level),[90] whereas Mount Everest (8,848 metres or 29,029 feet above local sea level) represents a deviation of 0.14%.[n 6][92]
In geodesy, the exact shape that Earth's oceans would adopt in the absence of land and perturbations such as tides and winds is called the geoid. More precisely, the geoid is the surface of gravitational equipotential at mean sea level (MSL).[93] Sea surface topography are water deviations from MSL, analogous to land topography.
Chemical composition
See also: Abundance of elements on Earth
Chemical composition of the crust[94][95]
Compound Formula Composition
Continental Oceanic
silica SiO
2 60.6% 50.1%
alumina Al
2O
3 15.9% 15.7%
lime CaO 6.41% 11.8%
magnesia MgO 4.66% 10.3%
iron oxide FeOT 6.71% 8.3%
sodium oxide Na
2O 3.07% 2.21%
potassium oxide K
2O 1.81% 0.11%
titanium dioxide TiO
2 0.72% 1.1%
phosphorus pentoxide P
2O
5 0.13% 0.1%
manganese oxide MnO 0.10% 0.11%
Total 100% 99.8%
Earth's mass is approximately 5.97×1024 kg (5,970 Yg). It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminum (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is estimated to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[96]
The most common rock constituents of the crust are nearly all oxides: chlorine, sulfur, and fluorine are the important exceptions to this and their total amount in any rock is usually much less than 1%. Over 99% of the crust is composed of 11 oxides, principally silica, alumina, iron oxides, lime, magnesia, potash, and soda.[97][96]
Internal structure
Main article: Structure of the Earth
Geologic layers of Earth[98]
Earth-cutaway-schematic-english.svg
Earth cutaway from core to exosphere. Not to scale.
Depth[99]
km Component layer Density
g/cm3
0–60 Lithosphere[n 7] —
0–35 Crust[n 8] 2.2–2.9
35–660 Upper mantle 3.4–4.4
660–2890 Lower mantle 3.4–5.6
100–700 Asthenosphere —
2890–5100 Outer core 9.9–12.2
5100–6378 Inner core 12.8–13.1
Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity.[100] The thickness of the crust varies from about 6 kilometres (3.7 mi) under the oceans to 30–50 km (19–31 mi) for the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, which is divided into independently moving tectonic plates.[101]
Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km (250 and 410 mi) below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[102] 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.[103] The radius of the inner core is about one-fifth of that of Earth. Density increases with depth, as described in the table on the right.
Heat
Main article: Earth's internal heat budget
The major heat-producing isotopes within Earth are potassium-40, uranium-238, and thorium-232.[104] At the center, the temperature may be up to 6,000 °C (10,830 °F),[105] and the pressure could reach 360 GPa (52 million psi).[106] Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's history, before isotopes with short half-lives were depleted, Earth's heat production was much higher. At approximately 3 Gyr, twice the present-day heat would have been produced, increasing the rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today.[107][108]
Present-day major heat-producing isotopes[107]
Isotope Heat release
W
/
kg isotope
Half-life
years Mean mantle concentration
kg isotope
/
kg mantle
Heat release
W
/
kg mantle
238U 94.6×10−6 4.47×109 30.8×10−9 2.91×10−12
235U 569×10−6 0.704×109 0.22×10−9 0.125×10−12
232Th 26.4×10−6 14.0×109 124×10−9 3.27×10−12
40K 29.2×10−6 1.25×109 36.9×10−9 1.08×10−12
The mean heat loss from Earth is 87 mW m−2, for a global heat loss of 4.42×1013 W.[109] 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.[110] More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans because the crust there is much thinner than that of the continents.[111]
Tectonic plates
Main article: Plate tectonics
Earth's major plates[112]
Shows the extent and boundaries of tectonic plates, with superimposed outlines of the continents they support
Plate name Area
106 km2
Pacific Plate
103.3
African Plate[n 9]
78.0
North American Plate
75.9
Eurasian Plate
67.8
Antarctic Plate
60.9
Indo-Australian Plate
47.2
South American Plate
43.6
Earth's mechanically rigid outer layer, the lithosphere, is divided into tectonic plates. These plates are rigid segments that move relative to each other at one of three boundaries types: at convergent boundaries, two plates come together; at divergent boundaries, two plates are pulled apart; and at transform boundaries, two plates slide past one another laterally. Along these plate boundaries, earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur.[113] 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.[114]
As the tectonic plates migrate, oceanic crust is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than 100 Ma old. The oldest oceanic crust is located in the Western Pacific and is estimated to be 200 Ma old.[115][116] By comparison, the oldest dated continental crust is 4,030 Ma,[117] although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to 4,400 Ma, indicating that at least some continental crust existed at that time.[49]
The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 Ma. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/a (3.0 in/year)[118] and the Pacific Plate moving 52–69 mm/a (2.0–2.7 in/year). At the other extreme, the slowest-moving plate is the South American Plate, progressing at a typical rate of 10.6 mm/a (0.42 in/year).[119]
Surface
Main articles: Earth's crust, Landform, and Extreme points of Earth
See also: Planetary surface and Geomorphology
Current Earth without water, elevation greatly exaggerated (click/enlarge to "spin" 3D-globe).
The total surface area of Earth is about 510 million km2 (197 million sq mi).[15] Of this, 70.8%,[15] or 361.13 million km2 (139.43 million sq mi), is below sea level and covered by ocean water.[120] Below the ocean's surface are much of the continental shelf, mountains, volcanoes,[86] oceanic trenches, submarine canyons, oceanic plateaus, abyssal plains, and a globe-spanning mid-ocean ridge system. The remaining 29.2%, or 148.94 million km2 (57.51 million sq mi), not covered by water has terrain that varies greatly from place to place and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest. The mean height of land above sea level is about 797 m (2,615 ft).[121]
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.[122] Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust.[123] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine.[124] Common carbonate minerals include calcite (found in limestone) and dolomite.[125]
Erosion and tectonics, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshape Earth's surface over geological time.[126][127]
The pedosphere is the outermost layer of Earth's continental surface and is composed of soil and subject to soil formation processes. The total arable land is 10.9% of the land surface, with 1.3% being permanent cropland.[128][129] Close to 40% of Earth's land surface is used for agriculture, or an estimated 16.7 million km2 (6.4 million sq mi) of cropland and 33.5 million km2 (12.9 million sq mi) of pastureland.[130]
Gravitational field
Main article: Gravity of Earth
Earth's gravity measured by NASA's GRACE mission, showing deviations from the theoretical gravity. Red shows where gravity is stronger than the smooth, standard value, and blue shows where it is weaker.
The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth's surface, gravitational acceleration is approximately 9.8 m/s2 (32 ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad, regional differences in Earth's gravitational field, known as gravity anomalies.[131]
Magnetic field
Main article: Earth's magnetic field
The main part of Earth's magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is, approximately, a dipole. The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05×10−5 T, with a magnetic dipole moment of 7.79×1022 Am2 at epoch 2000, decreasing nearly 6% per century.[132] 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.[133][134]
Magnetosphere
Main article: Magnetosphere
Diagram showing the magnetic field lines of 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 wind flows from left to right
The extent of Earth's magnetic field in space defines the magnetosphere. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the dayside of the magnetosphere, to about 10 Earth radii, and extends the nightside magnetosphere into a long tail.[135] Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic bow shock precedes the dayside magnetosphere within the solar wind.[136] Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.[137][138] 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,[139] and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.[140][141]
During magnetic storms and substorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing the aurora.[142]
Orbit and rotation
Rotation
Main article: Earth's rotation
Earth's rotation imaged by DSCOVR EPIC on 29 May 2016, a few weeks before a solstice.
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).[143] 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.[144][145]
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s.[4][n 10] Earth's rotation period relative to the precessing or moving mean March equinox (when the Sun is at 90° on the equator), is 86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s).[4] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[146]
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.[147][148]
Orbit
Main article: Earth's orbit
The Pale Blue Dot photo taken in 1990 by the Voyager 1 spacecraft showing Earth (center right) from nearly 6.0 billion km (3.7 billion mi) away, about 5.6 hours at light speed.[149]
Earth orbits the Sun at an average distance of about 150 million km (93 million mi) every 365.2564 mean solar days, or one sidereal year. This gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance to the Moon, 384,000 km (239,000 mi), in about 3.5 hours.[5]
The Moon and Earth orbit a common barycenter every 27.32 days relative to the background stars. When combined with the Earth-Moon system's common orbit around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon, and their axial rotations are all counterclockwise. Viewed from a vantage point above the Sun and Earth's north poles, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.44 degrees from the perpendicular to the Earth-Sun plane (the ecliptic), and the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[5][150]
The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.[151][n 11] This is the maximum distance at which Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.[151]
Earth, along with the Solar System, is situated in the Milky Way and orbits about 28,000 light-years from its center. It is about 20 light-years above the galactic plane in the Orion Arm.[152]
Axial tilt and seasons
Main article: Axial tilt § Earth
Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit
The axial tilt of Earth is approximately 23.439281°[4] with the axis of its orbit plane, always pointing towards the Celestial Poles. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes the seasonal change in climate, with summer in the Northern Hemisphere occurring when the Tropic of Cancer is facing the Sun, and in the Southern Hemisphere when the Tropic of Capricorn faces the Sun. In each instance, winter occurs simultaneously in the opposite hemisphere. During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and the days shorter.[153] 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.[154][155]
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.[156]
The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.[157] 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.[158]
In modern times, Earth's perihelion occurs around 3 January, and its aphelion around 4 July. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance causes an increase of about 6.8% in solar energy reaching Earth at perihelion relative to aphelion.[159][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.[160]
Earth-Moon system
Main articles: Orbit of the Moon and Satellite system (astronomy)
See also: Claimed moons of Earth, Quasi-satellite, Co-orbital moon, Co-orbital configuration, and Temporary satellite
Moon
Main articles: Moon and Lunar theory
Characteristics
Full moon as seen from Earth's Northern Hemisphere
Diameter 3,474.8 km
Mass 7.349×1022 kg
Semi-major axis 384,400 km
Orbital period 27d 7h 43.7m
The Moon is a relatively large, terrestrial, planet-like natural satellite, with a diameter about one-quarter of Earth's. It is the largest moon in the Solar System relative to the size of its planet, although Charon is larger relative to the dwarf planet Pluto.[161][162] The natural satellites of other planets are also referred to as "moons", after Earth's.[163] The most widely accepted theory of the Moon's origin, the giant-impact hypothesis, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth's crust.[41]
The gravitational attraction between Earth and the Moon causes tides on Earth.[164] 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.[165] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases.[166] Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm/a (1.5 in/year). Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs/yr—add up to significant changes.[167] During the Ediacaran period, for example, (approximately 620 Ma) there were 400±7 days in a year, with each day lasting 21.9±0.4 hours.[168]
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.[169] 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.[170][171]
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.[148] This allows total and annular solar eclipses to occur on Earth.[172]
Asteroids and artificial satellites
Main article: Near-Earth object
Tracy Caldwell Dyson viewing Earth from the ISS Cupola, 2010
Earth's co-orbital asteroids population consists of quasi-satellites, objects with a horseshoe orbit and trojans. There are at least five quasi-satellites, including 469219 Kamoʻoalewa.[173][174] A trojan asteroid companion, 2010 TK7, is librating around the leading Lagrange triangular point, L4, in Earth's orbit around the Sun.[175][176] The tiny near-Earth asteroid 2006 RH120 makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.[177]
As of April 2020, there are 2,666 operational, human-made satellites orbiting Earth.[8] There are also inoperative satellites, including Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked space debris.[n 3] Earth's largest artificial satellite is the International Space Station.[178]
Hydrosphere
Main article: Hydrosphere
Water typically evaporates over water surfaces like oceans and is transported to land via the atmosphere. Precipitation in the form of snow, rain, and more then brings it back to the surface. A system of rivers brings the water back to oceans and seas.
Water is transported to various parts of the hydrosphere via the water cycle.
The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from other planets in the Solar System. Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m (6,600 ft). The mass of the oceans is approximately 1.35×1018 metric tons or about 1/4400 of Earth's total mass. The oceans cover an area of 361.8 million km2 (139.7 million sq mi) with a mean depth of 3,682 m (12,080 ft), resulting in an estimated volume of 1.332 billion km3 (320 million cu mi).[179] If all of Earth's crustal surface were at the same elevation as a smooth sphere, the depth of the resulting world ocean would be 2.7 to 2.8 km (1.68 to 1.74 mi).[180] About 97.5% of the water is saline; the remaining 2.5% is fresh water.[181][182] Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.[183]
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.[184] 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.[185]
The average salinity of Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt).[186] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[187] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[188] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[189] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.[190]
Atmosphere
Main article: Atmosphere of Earth
Satellite image of Earth cloud cover using NASA's Moderate-Resolution Imaging Spectroradiometer
The atmospheric pressure at Earth's sea level averages 101.325 kPa (14.696 psi),[191] with a scale height of about 8.5 km (5.3 mi).[5] A dry atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules.[191] Water vapor content varies between 0.01% and 4%[191] but averages about 1%.[5] The height of the troposphere varies with latitude, ranging between 8 km (5 mi) at the poles to 17 km (11 mi) at the equator, with some variation resulting from weather and seasonal factors.[192]
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today.[61] This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O
2 into O
3. The ozone layer blocks ultraviolet solar radiation, permitting life on land.[193] 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.[194] 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, 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),[195] and life on Earth probably would not exist in its current form.[196]
Weather and climate
Main articles: Weather and Climate
Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (6.8 mi) of the surface. This lowest layer is called the troposphere. 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.[197]
Hurricane Felix seen from low Earth orbit, September 2007
Massive clouds above the Mojave Desert, February 2016
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°.[198] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[199]
The amount of solar energy reaching the Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C (0.7 °F) per degree of latitude from the equator.[200] 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.[201]
Further factors that affect a location's climates are its proximity to oceans, the oceanic and atmospheric circulation, and topology.[202] 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.[203] 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.[204] Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas.[205]
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.[197] 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.[206]
The commonly used Köppen climate classification system has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.[198] The Köppen system rates regions based on observed temperature and precipitation.[207] Surface air temperature can rise to around 55 °C (131 °F) in hot deserts, such as Death Valley, and can fall as low as −89 °C (−128 °F) in Antarctica.[208][209]
Upper atmosphere
This view from orbit shows the full moon partially obscured by Earth's atmosphere.
Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[194] 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.[210] Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100 km (62 mi) above Earth's surface, is a working definition for the boundary between the atmosphere and outer space.[211]
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.[212] 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.[213] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[214] 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.[215]
Life on Earth
Main article: Life
Fungi are one of the kingdoms of life on Earth.
A planet's life forms inhabit ecosystems, whose total forms the biosphere.[216] The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals.[217] 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 in humid lowlands at equatorial latitudes.[218] Estimates of the number of species on Earth today vary; most species have not been described.[219] Over 99% of all species that ever lived on Earth are extinct.[220][221]
A planet that can sustain life is termed habitable, even if life did not originate there. The distance of Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere, and magnetic field all contribute to the current climatic conditions at the surface.[222] Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.[223] Plants can take up nutrients from the atmosphere, soils and water. These nutrients are constantly recycled between different species.[224]
Extreme weather, such as tropical cyclones (including hurricanes and typhoons), occurs over most of Earth's surface and has a large impact on life in those areas. From 1980 to 2000, these events caused an average of 11,800 human deaths per year.[225] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, blizzards, floods, droughts, wildfires, and other calamities and disasters.[226] Human impact is felt in many areas due to pollution of the air and water, acid rain, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion.[227] Human activities release greenhouse gases into the atmosphere which cause global warming.[228] This is driving changes such as the melting of glaciers and ice sheets, a global rise in average sea levels, increased risk of drought and wildfires, and migration of species to colder areas.[229]
Human geography
Main articles: Human geography and World
The seven continents of Earth:[230]
North America
South America
Antarctica
Europe
Africa
Asia
Australia
vte
Earth's human population passed seven billion in the early 2010s,[231] and is projected to peak at around ten billion in the second half of the 21st century.[232] Most of the growth is expected to take place in sub-Saharan Africa.[232] Human population density varies widely around the world, but a majority live in Asia. By 2050, 68% of the world's population is expected to be living in urban, rather than rural, areas.[233] The Northern Hemisphere contains 68% of the world's land mass.[234] Partly due to the predominance of land mass, 90% of humans live in the Northern Hemisphere.[235]
It is estimated that one-eighth of Earth's surface is suitable for humans to live on—three-quarters of Earth's surface is covered by oceans, leaving one-quarter as land. Half of that land area is desert (14%),[236] high mountains (27%),[237] or other unsuitable terrains. States claim the planet's entire land surface, except for parts of Antarctica and a few other unclaimed areas.[238] Earth has never had a planetwide government, but the United Nations is the leading worldwide intergovernmental organization.[239]
The first human to orbit Earth was Yuri Gagarin on 12 April 1961.[240] In total, about 550 people have visited outer space and reached orbit as of November 2018, and, of these, twelve have walked on the Moon.[241][242] Normally, the only humans in space are those on the International Space Station. The station's crew, made up of six people, is usually replaced every six months.[243] The farthest that humans have traveled from Earth is 400,171 km (248,655 mi), achieved during the Apollo 13 mission in 1970.[244]
Natural resources and land use
Main articles: Natural resource and Land use
Land use in 2015 as a percentage of ice-free land surface[245]
Land use Percentage
Cropland 12–14%
Pastures 30–47%
Human-used forests 16–27%
Infrastructure 1%
Unused land 24–31%
Earth has resources that have been exploited by humans.[246] Those termed non-renewable resources, such as fossil fuels, are only replenished over geological timescales.[247] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[248] These deposits are used by humans both for energy production and as feedstock for chemical production.[249] 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.[250] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[251]
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.[252] In 2019, 39 million km2 (15 million sq mi) of Earth's land surface consisted of forest and woodlands, 12 million km2 (4.6 million sq mi) was shrub and grassland, 40 million km2 (15 million sq mi) were used for animal feed production and grazing, and 11 million km2 (4.2 million sq mi) were cultivated as croplands.[253] Of the 12–14% of ice-free land that is used for croplands, 2 percentage points were irrigated in 2015.[245] Humans use building materials to construct shelters.[254]
Humans and climate
See also: Climate change
Human activities, such as the burning of fossil fuels, emit greenhouse gases into the atmosphere of the Earth, altering its climate.[255][256][257] It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.[258] 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.[229]
Cultural and historical viewpoint
Main article: Earth in culture
Earthrise, taken in 1968 by William Anders, an astronaut on board Apollo 8
Human cultures have developed many views of the planet.[259] The standard astronomical symbol of Earth consists of a cross circumscribed by a circle, Earth symbol.svg,[260] representing the four corners of the world. Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity.[261] Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.[261] The Gaia hypothesis, developed in the mid-20th century, compared Earth's environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.[262][263][264] Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, emphasizing its beauty, uniqueness and apparent fragility.[265][266]
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.[267][268] 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, comparable to the other planets in the Solar System.[269]
It was only during the 19th century that geologists realized Earth's age was at least many millions of years.[270] Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth's age was established, proving the planet to be billions of years old.[271][272]
See also
Celestial sphere
Earth phase
Earth physical characteristics tables
Earth science
Outline of Earth
Table of physical properties of planets in the Solar System
Timeline of natural history
Timeline of the far future
Notes
All astronomical quantities vary, both secularly and periodically. The quantities given are the values at the instant J2000.0 of the secular variation, ignoring all periodic variations.
aphelion = a × (1 + e); perihelion = a × (1 – e), where a is the semi-major axis and e is the eccentricity. The difference between Earth's perihelion and aphelion is 5 million kilometers.—Wilkinson, John (2009). Probing the New Solar System. CSIRO Publishing. p. 144. ISBN 978-0-643-09949-4.
As of 4 January 2018, the United States Strategic Command tracked a total of 18,835 artificial objects, mostly debris. See: Anz-Meador, Phillip; Shoots, Debi, eds. (February 2018). "Satellite Box Score" (PDF). Orbital Debris Quarterly News. 22 (1): 12. Retrieved 18 April 2018.
Earth's circumference is almost exactly 40,000 km because the meter was calibrated on this measurement—more specifically, 1/10-millionth of the distance between the poles and the equator.
Due to natural fluctuations, ambiguities surrounding ice shelves, and mapping conventions for vertical datums, exact values for land and ocean coverage are not meaningful. Based on data from the Vector Map and Global Landcover Archived 26 March 2015 at the Wayback Machine datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of Earth's surface. The ice sheets of Antarctica and Greenland are counted as land, even though much of the rock that supports them lies below sea level.
If Earth were shrunk to the size of a billiard ball, some areas of Earth such as large mountain ranges and oceanic trenches would feel like tiny imperfections, whereas much of the planet, including the Great Plains and the abyssal plains, would feel smoother.[91]
Locally varies between 5 and 200 km.
Locally varies between 5 and 70 km.
Including the Somali Plate, which is being formed out of the African Plate. See: Chorowicz, Jean (October 2005). "The East African rift system". Journal of African Earth Sciences. 43 (1–3): 379–410. Bibcode:2005JAfES..43..379C. doi:10.1016/j.jafrearsci.2005.07.019.
The ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".—Aoki, S.; Kinoshita, H.; Guinot, B.; Kaplan, G. H.; McCarthy, D. D.; Seidelmann, P. K. (1982). "The new definition of universal time". Astronomy and Astrophysics. 105 (2): 359–61. Bibcode:1982A&A...105..359A.
For Earth, the Hill radius is {\displaystyle R_{H}=a\left({\frac {m}{3M}}\right)^{\frac {1}{3}}} R_{H}=a\left({\frac {m}{3M}}\right)^{\frac {1}{3}}, 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 {\displaystyle \left({\frac {1}{3\cdot 332,946}}\right)^{\frac {1}{3}}=0.01} \left({\frac {1}{3\cdot 332,946}}\right)^{\frac {1}{3}}=0.01.
Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% of the energy at aphelion.
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