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Earth

Earth (or "the Earth",                  Orbital characteristics
sometimes called "Terra"
or "Tellus" by science     Mean radius          149,597,870 km
fiction enthusiasts) is    Perihelion           0.983 AU
the third planet from the
Sun in our solar system    Aphelion             1.017 AU
and the only one in the    Eccentricity         0.01671022
universe known by us to be Orbital period       365.25636 days
inhabited by intelligent
life (there is continuing  Avg. Orbital Speed   29.7859 km/s
controversy about the      Inclination          0.00005
existence and nature of    Satellites           1 (the Moon)
extraterrestrial life).
The planet has one natural Satellite of         Sun
satellite, called the Moon             Physical characteristics
or Luna. It is the largest
and most massive of the    Equatorial diameter  12,756.3 km
terrestrial planets.       Surface area         5.10072×108 km2
(Compare to gas giant.)
                           Mass                 5.9742×1024 kg

The image to the right is  Mean density         5.515 g/cm3
of Africa, Antarctica, and
the Arabian Peninsula as   Surface gravity      9.78 m/s2
taken en route to the Moon Escape velocity      11.18 km/s
by Apollo 17 on December
7, 1972. Click here for a  Rotation period      23.9345 hours
larger image.              Axial tilt           23.45
                           Albedo               37-39%
                                                min  mean  max
                           Surface temperature
                                                184 K282 K 333 K
                                      Atmospheric characteristics
                           Pressure             101.325 kPa
                           nitrogen             78%
                           oxygen               21%
                           argon                1%
                           carbon dioxide
                           water vapor          trace

Physical characteristics

Structure

The interior of Earth, like that of the other terrestrial planets, is
chemically divided into an outer siliceous solid crust, with 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 an
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 the Earth's
surface is less than 100,000,000 years old.

Interior

The interior of the 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
        o 0-30/35 km -- Crust
   * 30/35-2900 km -- Mantle
        o 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).

The 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

The 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).

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-melted (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 5-35 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

Earth is the only place in the universe where we, humans, have reliably
observed life. The layer of life is called the biosphere,

The biosphere is divided into a number of biomes, or areas inhabited by a
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 richest
ones lie near the Equator.

Terrestrial organisms in temperate and arctic biomes have relatively small
amounts of total biomass, smaller energy budgets, and display prominent
adaptations to cold, including world-spanning migrations, social
adaptations, homeothermy, estivation and multiple layers of insulation. Some
theorists therefore believe that the Earth is poorly suited to life.

However, every part of the planet supports life, from the polar ice caps to
the Equator. Recent advances in microbiology have proven that microscopic
life lives inside rocks under the Earth's surface, and that the total mass
of microbial life in so-called "uninhabitable zones" may, in terms of sheer
biomass, outweigh all animal and plant life combined on the surface of the
Earth.

Oceans mediate the cold and distribute nutrients. The Antarctic krill,
Euphausia superba, for example, is generally considered to be the most
successful animal of the planet, with a biomass probably over 500 million
tonnes (c.f. human biomass of about 250 million tonnes).

Atmosphere

Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh
water [1]) and divides it into five oceans and seven continents. It 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. See also: Earth's atmosphere.

Hydrosphere

Earth is the only planet in our solar system, or even the known universe,
whose surface has liquid water. 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, the
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 the 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.

The Earth also suffers from the Chandler wobble.

The Moon

Diameter(km)     Mass (kg)     Mean Orbital Radius (km) Orbital Period

3,474.8      7.349 × 1022 384,400                  27Days,7hours,
                                                             43.7minutes

Earth is unique in its solar system in having a moon, called "the Moon" (or,
occasionally, "Luna"), which 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 may enable life by moderating the weather. Paleontological evidence
shows that Earth's axial tilt is stabilised by tidal interactions with its
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 the Earth, the same
apparent angular size as the Sun. This allows a total eclipse to occur on
Earth.

Also, the Moon is tidally locked: its rotation period is the same as the
time it takes to revolve around the Earth, meaning it always presents the
same face to the planet, seeming to disappear and reappear as the solar
terminator line moves around the moon.

The origin of the Moon is presently unknown, but one popular theory has it
that it was formed from the collision of a Mars-sized protoplanet into 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

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

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: Marianas Trench in the Pacific Ocean -10,924 m
   * Highest point: Mount Everest 8,850 m (1999 est.)

Natural resources

   * The 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 the Earth's crust by the action
     of erosion and plate tectonics. These ore bodies form concentrated
     sources for many metals and other useful elements.
   * The 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

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

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

In orbit about the Earth: 3 astronauts (November 1, 2003), on board the the
International Space Station.

The northernmost settlement in the world is Alert, Ellesmere Island, Canada.

Age structure:

   * 0-14 years: 1,818,803,078 (29.92%)
        o male: 932,832,913 (15.35%)
        o female: 885,970,165 (14.57%)
   * 15-64 years: 3,840,881,326 (63.19%)
        o male: 1,942,402,264 (31.95%)
        o female: 1,898,479,062 (31.23%)
   * 65 years and over: 419,090,130 (6.89%)
        o male: 184,072,470 (3.03%)
        o female: 235,017,660 (3.87%) (2000 est.)

Population growth rate: 1.3% (2000 est.)

Birth rate: 22 births/1,000 population (2000 est.)

Death rate: 9 deaths/1,000 population (2000 est.)

Sex ratio:

   * at birth: 1.05 male(s)/female
   * under 15 years: 1.05 male(s)/female
   * 15-64 years: 1.02 male(s)/female
   * 65 years and over: 0.78 male(s)/female
   * total population: 1.01 male(s)/female (2000 est.)

Infant mortality rate: 54 deaths/1,000 live births (2000 est.)

Life expectancy at birth:

   * total population: 64 years
   * male: 62 years
   * female: 65 years (2000 est.)

Total fertility rate: 2.8 children born/woman (2000 est.)

Government

The worldwide general international organization is United Nations.

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

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