Earth, also known as the Earth and Terra, is the planet on which humans live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body in the solar system that has conditions suitable for life, at least as known to modern science. The planet has a single natural satellite, the Moon, and it is generally believed to have formed around 4.5 billion years ago.

Physical characteristics

Main article: Geophysics

Structure

The interior of Earth, like that of the other terrestrial planets, is chemically divided into an outer siliceous solid crust, a highly viscous mantle, an outer core that is less viscous than the mantle, and an inner core. The planet is big enough to have the core differentiated into a liquid outer core, which gives rise to a weak magnetic field due to the convection of its electrically conductive material, and a solid inner core.

New material constantly finds its way to the surface through volcanoes and cracks in the ocean floors (see seafloor spreading). Much of Earth's surface is less than 100,000,000 years old; the very oldest parts of the crust are as much as 4.4 billion years old [1] .

Taken as a whole, Earth's composition by mass is:

34.6% Iron
29.5% Oxygen
15.2% Silicon
12.7% Magnesium
2.4% Nickel
1.9% Sulfur
0.05% Titanium

Interior

The interior of Earth reaches temperatures of 5270 K. The planet's internal heat was originally released during its accretion (see gravitational binding energy), and since then additional heat has continued to be generated by the decay of radioactive elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun.

0-60 km -- Lithosphere
- 0-30/35 km -- Crust
30/35-2900 km -- Mantle
- 100-700 km -- Asthenosphere
2900-5100 km -- Outer Core
5100-~6375 km -- Inner Core

The core

The outer core has a radius of ~3500 km. The inner core has a radius of ~1250 km.

The average density of Earth is 5,515 kg/m³, making it the densest planet in the Solar system. Since the average density of surface material is around 3000 kg/m³, this indicates that denser materials exist within the core. (see: planetary differentiation) It is thought that the core is largely composed of iron (80%), along with nickel and silicon; with other dense elements such as lead and uranium either being too rare to be significant or being felsic-seeking in nature (and thus concentrated in the crust rather than the core).

Earth was entirely molten about 4.6 billion years ago. Gravity would have caused denser substances to sink towards the center in a process called chemical differentiation, while less dense substances would have migrated to the crust.

The inner core is generally believed to be solid and to be composed entirely of iron and some nickel. Some believe it may be entirely composed of a single iron crystal. The inner core is surrounded by the outer core, which is believed to be liquid iron mixed with liquid nickel.

Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet, by ~2° per year (Comins DEU-p.82). It is generally believed that the rotation of the inner core (which is primarily composed of iron) creates the Earth's magnetic field. It is not known, exactly, why this occurs. (See also: dynamo theory)

Mantle

Earth's mantle extends to a depth of 2,900 km. The pressure, at the bottom of the mantle, is ~1.4 Matm (140 GPa). It is largely composed of substances rich in iron and magnesium. The melting point of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is plastic (semi-molten). The viscosity of the upper mantle ranges between 10²¹ and 10²⁴ Poise, depending on depth [2] . Thus, the upper mantle can only flow very slowly.

Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting point of iron rich substances are higher than pure iron. The core is composed almost entirely of pure iron, while iron rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-molten (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the center of the planet.

Crust

The crust ranges from 5 to 35 km in depth. It is composed of silicon-based rocks. The crust-mantle boundary occurs as two physically different events. Firstly, there is a discontinuity in the seismic velocity which is known as the Mohorovicic discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain none (below). The second event is a chemical discontinuity between ultramafic cumulates and tectonized hartzburgites which has been observed from parts of the oceanic crust that have been obducted.

Biosphere

Main Article: Life

Earth is the only place where life is known to exist. The planet's lifeforms are sometimes said to form a "biosphere". This biosphere is generally believed to have evolved about 3.5 billion years ago. The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the more populous biomes lie near the Equator.

Atmosphere

Main article: Earth's atmosphere

Earth has a relatively thick atmosphere composed of 78% nitrogen, 21% oxygen, and 1% argon, plus traces of other gases including carbon dioxide and water. The atmosphere acts as a buffer between Earth and the Sun. The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes. This is sometimes described as the "third atmosphere" to distinguish it from earlier atmospheric compositions.

Hydrosphere

Main article: Ocean

Earth is the only planet in our solar system whose surface has liquid water. Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh water [3] ) and divides it into five oceans and seven continents. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.

Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (Archaea) had colonized the oceans, the greenhouse effect failed, and Earth froze solid for 10 to 100 million years.

On other planets, such as Venus, gaseous water is cracked by solar ultraviolet, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. It is believed that this is the reason why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.

On Earth, a shield of ozone absorbs most of this energetic ultraviolet high in the atmosphere, reducing the cracking effect. The magnetosphere also shields the ionosphere from direct scouring by the solar wind.

Finally, vulcanism, aided by the Moon's tidal effects, continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone fields are subducted into magma and volcanically emitted as gaseous carbon dioxide and steam.

Earth also suffers from the Chandler wobble.

Earth in the Solar System

Earth rotates once a day about the axis connecting the North Pole and the South Pole. It revolves around the Sun, completing one revolution per year. Earth has one natural satellite, "the Moon", which revolves around Earth, taking some 28 days to complete one revolution. Viewed from Earth's North Pole, all these revolutions proceed in a counterclockwise manner.

The planes of the various revolutions are not precisely aligned: Earth's axis is tilted some 23.5 degrees against the Earth-Sun plane (which causes the seasons), and the Earth-Moon plane is tilted about 5 degrees against the Earth-Sun plane (otherwise there would be an eclipse every month).

The Hill sphere (sphere of influence) of the earth is about 1.5 Gm (930 thousand miles) in radius, within which one natural satellite (the Moon) comfortably orbits.

The Moon

Name	Diameter(km)	Mass (kg)	Mean Orbital Radius (km)	Orbital Period
Moon	3,474.8	7.349 × 10²²	384,400	27 Days, 7 hours, 43.7 minutes

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

The Moon's gravitation is the cause of the tides on Earth. The same effect has lead to its tidal locking: its rotation period is the same as the time it takes to revolve around Earth, meaning it always presents the same face to the planet.

As the Moon revolves around Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases: a dark part of the face is separated from a light part by the solar terminator line.

The Moon may enable life by moderating the weather. Paleontological evidence shows that Earth's axial tilt is stabilised by tidal interactions with the Moon. Without this stabilization, the rotational axis might be chaotically unstable, as it is with a sphere. If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result as one pole was continually heated and the other cooled. Planetologists who have studied the effect claim that this might kill all large animal and higher plant life. This remains a controversial subject, however, and further studies of Mars - which shares Earth's rotation period and axial tilt, but not its large moon or liquid core - may provide additional information.

The Moon is just far enough away to have, when seen from Earth, the same apparent angular size as the Sun. This allows a total eclipse to occur on Earth. (The Sun is 400 times larger, but the Moon is 400 times closer.)

The Moon's origin is unknown, but one popular theory is that it was formed from the collision of a Mars-sized protoplanet with the early Earth. This theory explains (among other things) the Moon's relative lack of iron and volatile elements. See Giant impact theory.

Earth also has at least one known co-orbital asteroid, 3753 Cruithne.

Geography

Main article: Geography

Physical map of Earth.

Map references:

Time Zones, Coordinates.

Biggest geographic subdivision

Continents, Oceans

Area:

total: 510.072 million km²
land: 148.94 million km²
water: 361.132 million km²
note: 70.8% of the world's surface is covered by water, 29.2% is exposed land

Land boundaries: the land boundaries in the world total 251,480.24 km (not counting shared boundaries twice)

Coastline: 356,000 km

Maritime claims: see United Nations Convention on the Law of the Sea

contiguous zone: 24 nautical miles (NM) claimed by most, but can vary
continental shelf: 200 m depth claimed by most or to depth of exploitation; others claim 200 NM or to the edge of the continental margin
exclusive fishing zone: 200 NM claimed by most, but can vary
exclusive economic zone: 200 NM claimed by most, but can vary
territorial sea: 12 NM claimed by most, but can vary
Note: boundary situations with neighboring states prevent many countries from extending their fishing or economic zones to a full 200 NM; 43 nations and other areas that are landlocked include Afghanistan, Andorra, Armenia, Austria, Azerbaijan, Belarus, Bhutan, Bolivia, Botswana, Burkina Faso, Burundi, Central African Republic, Chad, Czech Republic, Ethiopia, Holy See (Vatican City), Hungary, Kazakhstan, Kyrgyzstan, Laos, Lesotho, Liechtenstein, Luxembourg, Malawi, Mali, Moldova, Mongolia, Nepal, Niger, Paraguay, Rwanda, San Marino, Slovakia, Swaziland, Switzerland, Tajikistan, The Republic of Macedonia, Turkmenistan, Uganda, Uzbekistan, West Bank, Zambia, Zimbabwe

Climate

Main article: Climate model

Two large areas of polar climates separated by two rather narrow temperate zones from a wide equatorial band of tropical to subtropical climates. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter.

Terrain

Elevation extremes: (measured relative to sea level)

Lowest point on land: Dead Sea −408 m
Lowest point overall: Mariana Trench in the Pacific Ocean −10,924 m
Highest point: Mount Everest 8,850 m (1999 est.)

Natural resources

Main article: Natural resources

Earth's crust contains large deposits of fossil fuels: (coal, oil, natural gas, methane clathrate). 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 ore bodies form concentrated sources for many metals and other useful elements.
Earth's biosphere produces many useful biological products, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.

Some of these resources, such as fossil 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: 10%
permanent crops: 1%
permanent pastures: 26%
forests and woodland: 32%
other: 31% (1993 est.)

Irrigated land: 2,481,250 km² (1993 est.)

Natural hazards

Large areas are subject to extreme weather such as (tropical cyclones), hurricanes,or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, floods, droughts, and other calamities and disasters.

Environment - current issues

Main article: Environmentalism

Large areas are subject to overpopulation, industrial disasters such as pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, soil degradation, soil depletion, erosion, and introduction of invasive species.

Human population

Earth at night, composite of pictures taken between October 1994 and March 1995.

Nearly all humans live on Earth: 6,327,152,352 inhabitants (November 1, 2003 est.)

A few humans are in orbit around Earth on board the International Space Station, with others traveling briefly above the atmosphere.

The northernmost settlement in the world is Alert, Ellesmere Island, Canada. The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole.

Government

The worldwide general international organization is the United Nations.

Administrative divisions: 267 nations, dependent areas, other, and miscellaneous entries

Earth has often been personified as a deity, in particular a goddess. See Gaea and Mother Earth. In Norse mythology, Earth was the son of Nott and Annar.

Earth has also been described as a massive spaceship, with a life support system that requires maintenance. See Spaceship Earth.

Since Earth is rather large, it is not immediately obvious to the naked eye that it is spherical. Because of this, in the past it was sometimes thought that Earth was in fact flat. See flat Earth.

Reference

Discovering the Essential Universe (Second Edition) by Comins (2001)

The Solar System

(For other objects and regions, see: List of solar system objects, Astronomical objects)