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Earth 
Famous "Blue Marble" photograph of Earth, taken from Apollo 17. |
| Orbital characteristics |
| Epoch J2000 |
| Aphelion |
152,097,701 km
1.0167103335 AU |
| Perihelion: |
147,098,074 km
0.9832898912 AU |
| Semi-major axis: |
149,597,887.5 km
1.0000001124 AU |
| Eccentricity: |
0.016710219 |
| Orbital period: |
365.256366 days
1.0000175 yr |
| Avg. orbital speed: |
29.783 km/s
107,218 km/h |
| Inclination: |
Reference (0)
7.25° to Sun's equator |
| Longitude of ascending node: |
348.73936° |
| Argument of perihelion: |
114.20783° |
| Satellites: |
1 (the Moon) |
| Physical characteristics |
| Mean radius: |
6,371.0 km [2] |
| Equatorial radius: |
6,378.1 km [1] |
| Polar radius: |
6,356.8 km [1] |
| Flattening: |
0.0033528 [1] |
| Circumference: |
40,075.02 km (equatorial)
40,007.86 km (meridional)
40,041.47 km (mean) |
| Surface area: |
510,065,600 km²
148,939,100 km² land (29.2 %)
361,126,400 km² water (70.8 %) |
| Volume: |
1.0832073×1012 km³ |
| Mass: |
5.9736×1024 kg |
| Mean density: |
5,515.3 kg/m³ |
| Equatorial surface gravity: |
9.780327 m/s²[3]
0.99732 g |
| Escape velocity: |
11.186 km/s
40,270 km/h
|
| Sidereal rotation period: |
0.997258 d
23h 56m 04.09054s[3] |
| Rotation velocity at equator: |
465.11 m/s |
| Axial tilt: |
23.439281° |
| Albedo: |
0.367 |
Surface temp.:
Kelvin
Celsius |
| min |
mean |
max |
| 185 K |
287 K |
331 K |
| -88.3 °C |
14 °C |
57.7 °C |
|
| Adjectives: |
Terrestrial, Terran, Telluric, Tellurian, Earthly |
| Atmosphere |
| Surface pressure: |
101.3 kPa (MSL) |
| Composition: |
78.08% N2
20.95% O2
0.93% Argon
0.038% Carbon dioxide
Trace water vapor (varies with climate) |
Earth (IPA: /ɜrθ/) is the third
planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both
diameter and mass. It is also referred to as the Earth,
Planet Earth, Gaia, Terra,[4] and "the
World".
Home to millions of species[5] including humans, Earth is the only place in the universe where life is known to have originated. Scientific
evidence indicates that the planet formed 4.54 billion years[6][7][8][9] ago, and life appeared on its surface within a billion years. Since then, Earth's
biosphere has significantly altered the atmosphere
and other abiotic conditions on the planet, enabling the proliferation of
aerobic organisms as well as the formation of the ozone
layer which, together with Earth's magnetic field, blocks harmful radiation,
permitting life on land.
Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's
surface.[10][11] Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.
Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times
it rotates about its axis. This length of time is a sidereal year, which is equal to
365.26 solar days.[12]
The Earth's axis of rotation is tilted 23.4°[13] away from the perpendicular to its orbital plane, producing seasonal variations on the planet's surface with a period of one
tropical year. Earth's only known natural
satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. A cometary bombardment during the early history of the planet played a role in the formation of the
oceans.[14] Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the Earth's orbit, caused by the gravitational influence of other planets, are
believed to have given rise to the ice ages that have intermittently covered significant
portions of Earth's surface in glacial sheets.
History
-
Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the
Solar System formed 4.54 billion years ago[6] out of the solar nebula, a
disk-shaped mass of dust and gas left over from the formation of the Sun. Initially molten, the
outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed
soon afterwards, possibly as the result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass[15] impacting the Earth in a glancing blow.[16] Some of this object's mass merged with the Earth and a portion was ejected into space, but enough
material survived to form an orbiting moon.
Outgassing and volcanic activity produced the primordial atmosphere. Condensing
water vapor, augmented by ice delivered by comets, produced the oceans.[14] The highly energetic chemistry is believed to have
produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life existed.[17]
The development of photosynthesis allowed the Sun's energy to be harvested directly by
life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of
ozone (a form of molecular oxygen [O3]) in the upper
atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[18] True multicellular organisms formed as cells within colonies
became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by
the ozone layer, life colonized the surface of Earth.[19]
As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents
migrated across the surface, occasionally combining to form a supercontinent. Roughly
750 million years ago (mya), the earliest known supercontinent, Rodinia, began to
break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally
Pangaea, which broke apart 180 mya.[20]
Since the 1960s, it has been hypothesized that severe glacial action between 750 and
580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice.
This hypothesis has been termed "Snowball Earth", and is of particular interest because
it preceded the Cambrian explosion, when multicellular life forms began to
proliferate.[21]
Following the Cambrian explosion, about 535 mya, there have been five
mass extinctions.[22] The last extinction event occurred 65 mya, when a meteorite collision probably triggered the
extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such
as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has
diversified, and several mya, an African ape-like animal gained the ability to stand upright.[23] This enabled tool use and encouraged communication that provided the nutrition
and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the
Earth in a short time span as no other life form had,[24]
affecting both the nature and quantity of other life forms.
The present pattern of ice ages began about 40 mya, then intensified during the
Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of
glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.[25]
Composition and structure
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a
gas giant like Jupiter. It is the largest of the four solar
terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest
surface gravity and the strongest magnetic
field.[26]
Shape
-
Size comparison of inner planets (left to right):
Mercury,
Venus, Earth, and
Mars
The Earth's shape is very close to an oblate spheroid — a rounded shape with a bulge
around the equator — although the precise shape (the geoid)
varies from this by up to 100 meters.[27] The
average diameter of the reference spheroid is about 12,742 km. More approximately the distance is
40,000 km/π because the meter was originally defined as
1/10,000,000 of the distance from the equator to the north pole through
Paris, France.[28]
The rotation of the Earth creates the equatorial
bulge so that the equatorial diameter is 43 km larger than the pole to
pole diameter.[29] The largest local
deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local
sea level) and the Mariana Trench (10,911 m below
local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed
in billiard balls.[30] Because of the bulge, the feature farthest from the center of the Earth is actually
Mount Chimborazo in Ecuador.[31]
Chemical composition
- See also: Abundance of elements on
Earth
The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of
iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur
(2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to
mass segregation, the core region is believed to be primarily composed of iron (88.8%),
with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[32]
The geochemist F. W. Clarke calculated that a little more than 47% of the
Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust
are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock
is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica
functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a
computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table
at right.) All the other constituents occur only in very small quantities.[33]
Internal structure
-
Earth cutaway from core to exosphere. Not to scale.
The interior of the Earth, like that of the other terrestrial planets, is
chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous
than the mantle, and a solid inner core. The crust is separated from the mantle by the
Mohorovičić discontinuity, and the thickness of the crust varies: averaging
6 km under the oceans and 30–50 km on the continents.[34]
The geologic component layers of the Earth[35] are at
the following depths below the surface:[36]
| Depth |
Layer |
Density
g/cm³ |
| Kilometers |
Miles |
| 0–60 |
0–37 |
Lithosphere (locally varies between 5 and 200 km) |
— |
| 0–35 |
0–22 |
... Crust (locally varies between 5 and 70 km) |
2.2–2.9 |
| 35–60 |
22–37 |
... Uppermost part of mantle |
3.4–4.4 |
| 35–2890 |
22–1790 |
Mantle |
3.4–5.6 |
| 100–700 |
62–435 |
... Asthenosphere |
— |
| 2890–5100 |
1790–3160 |
Outer core |
9.9–12.2 |
| 5100–6378 |
3160–3954 |
Inner core |
12.8–13.1 |
The internal heat of the planet is most likely produced by the radioactive decay of potassium-40, uranium-238 and thorium-232
isotopes. All three have half-life decay periods of
more than a billion years.[37] At the center of the
planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[38] 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.[39]
Tectonic plates
-
A map illustrating the Earth's major plates.
According to plate tectonics theory, the outermost part of the Earth's interior is made up of two layers: the lithosphere, comprising the crust, and the solidified uppermost
part of the mantle. Below the lithosphere lies the asthenosphere, which forms the inner part of the mantle. The asthenosphere behaves like a superheated and
extremely viscous liquid.[40]
The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of
three types of plate boundaries: convergent, divergent and transform. The last occurs where two plates
move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these
plate boundaries.[41]
The main plates are:[42]
Notable minor plates include the Indian Plate, 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 actually fused with Indian Plate between 50 and 55 million years ago. The
fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of
75 mm/yr[43] (3.0 in/yr) and the
Pacific Plate moving 52–69 mm/yr (2.1–2.7 in/yr). At the other extreme, the
slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about
21 mm/yr (0.8 in/yr).[44]
Surface
-
The Earth's terrain varies greatly from place to place. About 70.8%[45] of the surface is covered by water,
with much of the continental shelf below sea level. The submerged surface has
mountainous features, including a globe-spanning mid-ocean ridge system, as well as
undersea volcanoes,[29] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.
The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady
weathering from precipitation, thermal
cycles, and chemical effects. Glaciation, coastal
erosion, the build-up of
