Encyclopedia
Earth is the third
planet in the
solar system in terms of distance from the
Sun, and the fifth largest. It is also the largest of its
planetary system's
terrestrial planets, making it the largest solid body in the solar system, and it is the only place in the
universe known to support life. Accepted scientific theory states that the Earth was formed around 4.57 billion years ago and its largest
natural satellite, the
Moon, was orbiting it shortly thereafter, around 4.53 billion years ago.
Since it formed, the Earth has changed through geological and biological processes that have hidden traces of the original conditions. The outer surface is divided into several
tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid
Earth mantle and an iron core that generates a
magnetic field. The
atmospheric conditions have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt water oceans, and the remainder consists of continents and islands.
There is significant interaction between the Earth and its space environment. The relatively large moon provides ocean
tides and has gradually modified the length of the planet's rotation period. A
cometary bombardment during the early history of the planet is believed to have played a role in the formation of the oceans. Later,
asteroid impacts are understood to have caused significant changes to the surface environment. Changes in the orbit of the planet may also be responsible for the
ice ages that have covered significant portions of the surface in glacial sheets.
The Earth's only natural orbiting body is the Moon, although the asteroid
Cruithne has been erroneously described as such. Cruithne was discovered in 1986 and follows an elliptical orbit around the Sun at about the same average orbital radius as the Earth. However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit around the Sun that avoids close proximity with the Earth.
Lexicography
In English usage, the name can be capitalized or spelled in lowercase interchangeably, both when used absolutely or prefixed with "the" . Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth", so as to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb . Oxford Spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely and lowercase when preceded by "the" . The term almost exclusively exists in lowercase when appearing in common phrases, even without "the" preceding it .
Terms that refer to the Earth can use the
Latin root
terr-, as in
terraform and terrestrial. An alternative Latin root is
tellur-, which is used in words such as tellurian and
tellurium. Both terms derive from
terra and
tellus respectively, which are Latin words meaning "earth". Scientific terms such as
geography,
geocentric and geothermal use the Greek prefix
geo- , from
ge . In many science fiction books and video games, Earth is referred to as Terra or Gaia.
Astronauts refer to the Earth as "Terra Firma".
The English word "earth" has cognates in many modern and ancient languages. Examples in modern tongues include
aarde in
Dutch and
Erde in
German. The root has cognates in extinct languages such as
ertha in Old Saxon and
ert in Middle Irish, derived from the
Old English eorđe. All of these words derive from the Proto-Indo-European base *er-.
Several
Semitic languages have words for "earth" similar to those in
Indo-European languages.
Arabic has
aard;
Akkadian,
irtsitu;
Aramaic,
araa; Phoenician,
erets ; and
Hebrew, ??? . The etymological connection between the words in Indo-European and Semitic languages are uncertain, though, and may simply be coincidence.
The standard name for
people from Earth is Earthling, although Terran, Gaian, and Earther are alternate names that have been used in
Science Fiction.
Words for Earth in other languages include:
?????? ,
Maa ,
pamînt ,
föld ,
zemlja ,
diqiu ,
deiqao ,
jigu ,
Bumi ,
chikyuu ,
Jorden , and
dunia .
Symbol
The
astronomical symbol and
astrological symbol for Earth consists of a circled cross , the arms of the cross representing a
meridian and the
equator. A variant of the symbol has the cross atop the circle .
History
Based on the available evidence, scientists have been able to reconstruct detailed information about the planet's past. Earth is believed to have formed around 4.57 billion years ago out of the
solar nebula, along with the Sun and the other planets. Initially molten, the outer layer of the planet cooled when water began accumulating in the atmosphere when the planet was about half its current radius, resulting in the solid crust. The moon formed soon afterwards, possibly as the result of the impact with a Mars-sized object known as Theia. Outgassing and
volcanic activity produced the primordial atmosphere; condensing
water vapor, augmented by ice delivered by
comets, produced the oceans. 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 lived.
The development of
photosynthesis allowed the sun's energy to be harvested directly; the resultant
oxygen accumulated in the atmosphere and gave rise to the
ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called
eukaryotes. Cells within colonies became increasingly specialized, resulting in true multicellular organisms. Aided by the absorption of harmful
ultraviolet radiation by the ozone layer, life colonized the surface of Earth.
Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, occasionally combining to form a supercontinent. Roughly 750 million years ago , 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.
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 lifeforms began to proliferate.
Since the
Cambrian explosion, about 535 mya, there were five
mass extinctions. The last occurred 65 mya, when a meteorite collision probably triggered the extinction of the
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, a small African ape gained the ability to stand upright. 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 timespan as no other life form had, affecting both the nature and quantity of other life forms, and the global climate.
Shape
The Earth's shape is very close to an
oblate spheroid, although the precise shape varies from this by up to 100 meters . The average diameter of the reference spheroid is approximately 12,742 km . The
rotation of the Earth causes the
equator to bulge out slightly so that the equatorial diameter is 43 km larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are
Mount Everest and the
Mariana Trench . Hence compared to a perfect
ellipsoid, the Earth has a
tolerance of about one part in about 584, or 0.17%. For comparison, this is less than the 0.22% tolerance allowed in
billiard balls. Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in
Ecuador.
Composition
The
mass of the Earth is approximately 5980
yottagrams . It is composed mostly of iron , oxygen , silicon , magnesium , nickel , calcium and aluminium .
Internal structure
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 geologic component layers of the Earth are at the following depths below the surface:
| Depth | Layer |
|---|
| Kilometres | Miles |
|---|
| 0–60 | 0–37 | Lithosphere |
| 0–35 | 0–22 | ... Crust |
| 35–60 | 22–37 | ... Uppermost part of mantle |
| 35–2890 | 22–1790 | Mantle |
| 100–700 | 62–435 | ... Asthenosphere |
| 2890–5100 | 1790–3160 | Outer core |
| 5100–6378 | 3160–3954 | Inner core |
Tectonic plates
According to
plate tectonics theory currently accepted by the vast majority of scientists working in this area, 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 comprises the inner, viscous part of the mantle. The mantle behaves like a superheated and extremely viscous liquid.
The lithosphere essentially
floats on the asthenosphere. The lithosphere is broken up into what are called
tectonic plates. These plates move in relation to one another at one of three types of plate boundaries:
convergent, divergent, and transform.
Earthquakes,
volcanic activity,
mountain-building, and
oceanic trench formation occur along plate boundaries.
The main plates are
...
, covering the
Pacific Ocean - Oceanic plate
Notable minor plates include the
Indian Plate, the
Arabian Plate, the
Caribbean Plate, the
Nazca Plate and the
Scotia Plate.
Surface
The Earth's
terrain varies greatly from place to place. About 70% of the surface is covered by water, with much of the
continental shelf below sea level. If all of the land on Earth were spread evenly, then water would rise to an altitude of more than 2500 metres . The remaining 30% not covered by water consists of
mountains,
deserts,
plains,
plateaus, etc.
Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 × 10
9 acres of cropland and 8.4 × 10
9 acres of pastureland.
Extremes
Elevation extremes:Hydrosphere
The abundance of water on Earth is a unique feature that distinguishes our "Blue Planet" from others in the solar system. Approximately 70.8 percent of the Earth is covered by water and only 29.2 percent is terra firma.
The Earth's hydrosphere consists chiefly of the
oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. The average depth of the oceans is 3,794 m , more than five times the average height of the continents. The mass of the oceans is approximately 1.35 × 10^18 tonnes, or about 1/4400 of the total mass of the Earth.
Atmosphere
The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the
troposphere. Further up, the atmosphere is usually divided into the
stratosphere,
mesosphere, and thermosphere. Beyond these, the exosphere thins out into the
magnetosphere . An important part of the atmosphere for life on Earth is the
ozone layer.
The
atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 6 km. It is 78%
nitrogen and 21%
oxygen, with trace amounts of other gaseous molecules such as water vapor. The atmosphere protects the Earth's life forms by absorbing
ultraviolet solar radiation, moderating temperature, transporting water vapor, and providing useful gases. The atmosphere is one of the principal components in determining
weather and
climate.
Climate
The most prominent features of the Earth's climate are its two large polar regions, two narrow temperate zones, and a wide
equatorial
tropical region. Precipitation patterns vary widely, ranging from several metres of water per year to less than a millimetre.
Ocean currents are important factors in determining climate, particularly the spectacular
thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions.
Pedosphere
The
pedosphere is the outermost layer of the Earth that is composed of
soil and subject to soil formation processes. It exists at the interface of the
lithosphere,
atmosphere,
hydrosphere and
biosphere.
Biosphere
The planet's lifeforms are sometimes said to form a "
biosphere". This biosphere is generally believed to have begun
evolving about 3.5 billion years ago. Earth is the only place in the universe where life is absolutely known to exist, and some scientists believe that biospheres might be rare.
The biosphere is divided into a number of biomes, inhabited by broadly similar
flora and
fauna. On land, biomes are separated primarily by
latitude and height above the sea level. Terrestrial biomes lying within the
Arctic,
Antarctic Circle or in high altitudes are relatively barren of
plant and
animal life, while most of the more populous biomes lie near the
Equator.
Natural resources
- Earth's crust contains large deposits of fossil fuels: . These deposits are used by humans both for energy production and as feedstock for chemical production.
- Mineral ore bodies have been formed in Earth's crust by the action of erosion and plate tectonics. These bodies form concentrated sources for many metals and other useful elements.
- Earth's biosphere produces many useful biological products, including food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.
Some of these resources, such as
mineral fuels, are difficult to replenish on a short time scale, called
non-renewable resources. The exploitation of non-renewable resources by human
civilization has become a subject of significant controversy in modern environmentalism movements.
Land use
- Arable land: 13.13% However, this is a controversial subject, and further studies of Mars—which shares Earth's rotation period
- time [i]
...
and
axial tilt, but not its large moon or liquid core—may settle the matter.
Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent angular size as the Sun . This allows total
eclipses and annular eclipses to occur on Earth.
ge:Earth-Moon2.jpg|thumb|center|800px|The relative sizes of and distance between Earth and Moon, to scale]]
The most widely accepted theory of the Moon's origin, the
giant impact theory, states that it was formed from the collision of a Mars-size protoplanet with the early Earth. This hypothesis explains the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.
Earth has at least two co-orbital satellites, the
asteroids
