The Earth
A
true-color NASA satellite mosaic of Earth.
Artistic rendition
(highly exaggerated) of a Foucault pendulum showing that the Earth is not stationary, but
rotates.
Introduction
The Earth in the Solar system
The Earth in the Solar system
The Solar
System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular
cloud approximately 4.6
billion years ago.
The vast majority of the system's mass is in the Sun. Of the
many objects thatorbit the Sun, most of the mass is contained within eight relatively
solitary planets whose orbits are almost circular and
lie within a nearly flat disc called the ecliptic plane.
The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of
rock and metal.
The four outer planets, the gas giants,
are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are
composed mainly of hydrogen and helium; the two outer most planets, Uranus and Neptune, are
composed largely of ices, such as
water, ammonia andmethane,
and are often referred to separately as "ice giants".
The Solar System is also home to a number of regions
populated by smaller objects. The asteroid
belt, which lies between Mars and Jupiter, is similar to the terrestrial
planets as it is composed mainly of rock and metal. Beyond Neptune 's
orbit lie the Kuiper belt and scattered
disc; linked populations of trans-Neptunian objects composed mostly of ices such as water, ammonia and methane.
Within these populations, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have
been rounded by their own gravity, and are thus termed dwarf
planets.In addition to thousands of small bodies in those two regions, several dozen of
which are considered dwarf-planet candidates, various other small body
populations including comets, centaurs
and interplanetary dust freely travel between regions. Six of
the planets and three of the dwarf planets are orbited by natural
satellites, usually termed
"moons" after Earth's Moon. Each of the outer
planets is encircled by planetary
rings of dust and other
particles.
The planets of Solar system
The Earth
Earth (or the Earth) is the third planet from
the Sun, and the densest and fifth-largest of the eight planets
in the Solar System. It is
also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world, the Blue Planet, or by its Latin
name, Terra.
Earth formed approximately 4.54 billion years ago by accretion from the solar nebula, and life appeared on its surface within one billion
years. The planet is home to millions of species, including humans. 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 solar radiation,
permitting life on land. The physical properties of the
Earth, as well as itsgeological history and orbit, have allowed life
to persist. The planet is expected to continue supporting life for another 500 million to 2.3 billion years.
Earth's crust is divided into several rigid
segments, or tectonic plates,
that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by
salt water oceans, with the remainder consisting of continents and islands
which together have many lakes and other sources of water that contribute to
the hydrosphere.
Earth's poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap). The planet'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
space, especially the Sun and the Moon. At present, Earth orbits the Sun
once every 366.26 times it rotates about its own axis, which is equal to 365.26 solar days, or one sidereal year. The
Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's
surface with a period of one tropical year (365.24 solar days). 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. Between approximately 3.8 billion and4.1 billion years ago, numerous asteroid impacts during the Late Heavy Bombardment caused
significant changes to the greater surface environment.
Both the mineral resources of the planet and the
products of the biosphere contribute
resources that are used to support a global human population. These inhabitants are grouped into about
200 independent sovereign states (193 United Nations recognized
sovereign states), which interact through diplomacy, travel, trade, and
military action. Human cultures have developed many views of the
planet, including personification as a deity, a belief in a flat Earth or in the Earth as the center of the
universe, and a modern perspective of the world as an integrated environment that requires stewardship.
Earth, our home planet, is the only planet in our
solar system known to harbor
life: life that is incredibly diverse. All the things we need to survive exist
under a thin layer of atmosphere that separates us from the cold, airless void
of space.
Earth is made up of complex, interactive systems that create
a constantly changing world that we are striving to understand. From the
vantage point of space we are able to observe our planet globally, using
sensitive instruments to understand the delicate balance among its oceans, air,
land and life. Satellite observations help study and predict weather, drought,
pollution, climate change and many other phenomena that affect the environment,
economy and society.
Earth is the third planet from the sun and the fifth largest in our solar
system. Earth's diameter is just a few hundred kilometers larger than that of Venus.
The four seasons are a result of Earth's axis of rotation
being tilted 23.45 degrees with respect to the plane of Earth's orbit around
the sun. During part of the year, the northern hemisphere is tilted toward the
sun and the southern hemisphere is tilted away, producing summer in the north
and winter in the south. Six months later, the situation is reversed. During
March and September, when spring and fall begin in the northern hemisphere,
both hemispheres receive roughly equal amounts of solar illumination.
Earth's global ocean, which covers nearly 70% of the
planet's surface, has an average depth of about 4 km (2.5 miles). Fresh water
exists in the liquid phase only within a narrow temperature span: 32 to 212
degrees Fahrenheit (0 to 100o Celsius). This span is especially
narrow when contrasted with the full range of temperatures found within the
solar system. The presence and distribution of water vapor in the atmosphere is
responsible for much of Earth's weather.
We are enveloped by an atmosphere that consists of 78%
nitrogen, 21% oxygen and 1% other ingredients. The atmosphere affects Earth's
long-term climate and short-term local weather, shields us from much of the
harmful radiation coming from the sun and protects us from meteors as well: most of which burn up before
they can strike the surface as meteorites. Earth-orbiting satellites have
revealed that the upper atmosphere actually swells by day and contracts by
night due to solar heating during the day and cooling at night.
Our planet's rapid rotation and molten nickel-iron core give
rise to a magnetic field, which the solar wind distorts into a teardrop shape
in space. (The solar wind is a stream of charged particles continuously ejected
from the sun.) The Earth's magnetic field does not fade off into space, but has
definite boundaries. When charged particles from the solar wind become trapped
in Earth's magnetic field, they collide with air molecules above our planet's
magnetic poles. These air molecules then begin to glow, and are known as the
aurorae -- the northern and southern lights.
Earth's lithosphere, which includes the crust (both
continental and oceanic) and the upper mantle, is divided into huge plates that
are constantly moving. For example, the North American plate moves west over
the Pacific Ocean basin, roughly at a rate
equal to the growth of our fingernails. Earthquakes result when plates grind
past one another, ride up over one another, collide to make mountains, or split
and separate. The theory of motion of the large plates of the lithosphere is
known as plate tectonics. Developed within the last 40 years, this explanation
has unified the results of centuries of study of our planet.
How Earth Got its Name
The name Earth is at least 1,000 years old. All of the
planets, except for Earth, were named after Greek and Roman gods and goddesses.
However, the name Earth is an English/German word, which simply means the
ground: eor(th)e and ertha (Old English) and erde (German).
The modern English noun earth developed from Middle English erthe (recorded in 1137), itself from Old English eorthe (dating from before 725), ultimately
deriving from Proto-Germanic *erthō. Earth has cognates in all other Germanic languages, including Dutch aarde, German Erde, and Swedish, Norwegian, and Danish jord. In Germanic paganism,
the goddess Jörð in Norse mythology (mother of the god Thor) was named after and personified as
the Earth.
In general English usage, the name earth can be capitalized or spelled in
lowercase interchangeably, either when used absolutely or prefixed with
"the" (i.e. "Earth", "the Earth",
"earth", or "the earth"). Many deliberately spell the name
of the planet with a capital, both as "Earth" or "the
Earth". This is to distinguish it as a proper noun, distinct from the
senses of the term as a count noun or verb (e.g. referring to soil, the ground,
earthing in the electrical sense, etc.). 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 (e.g. Earth's atmosphere) and lowercase when preceded by
"the" (e.g. the atmosphere of the earth). The term almost exclusively
exists in lowercase when appearing in common phrases, even without
"the" preceding it (e.g. "It does not cost the earth.", "What
on earth are you doing?").
Chronology
The earliest dated Solar System
material was formed 4.5672 ± 0.0006 billion years ago, and by
4.54 billion years ago (within an uncertainty of 1%) the Earth and
the other planets in the Solar System had formed out of the solar nebula-a
disk-shaped mass of dust and gas left over from the formation of the Sun. This
assembly of the Earth through accretion was thus largely completed within
10–20 million years. 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 shortly thereafter, 4.53 billion years ago.
The current consensus model for the
formation of the Moon is the giant impact hypothesis, in which the Moon was created when a
Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass impacted the Earth
in a glancing blow. In this
model, some of this object's mass would have merged with the Earth and a
portion would have been ejected into space, but enough material would have been
sent into orbit to coalesce into the Moon.
Outgassing and volcanic activity produced the primordial
atmosphere of the Earth. Condensing water vapor,
augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans. The newly formed Sun was
only 70% of its present luminosity, yet
evidence shows that the early oceans remained liquid - a contradiction dubbed
the faint young Sun paradox. A combination of greenhouse gases and higher levels of solar activity served to raise the Earth's surface
temperature, preventing the oceans from freezing over. By 3.5 billion years
ago, the Earth's magnetic field was established, which helped prevent the
atmosphere from being stripped away by the solar wind.
Two major models have been proposed for
the rate of continental growth: steady
growth to the present-day and
rapid growth early in Earth history. Current research shows that the second
option is most likely, with rapid initial growth of continental crust followed
by a long-term steady continental area. On time scales lasting
hundreds of millions of years, the surface continually reshaped as continents
formed and broke up. The continents migrated across the surface, occasionally
combining to form asupercontinent.
Roughly 750
million years ago (Ma), one of the earliest known
supercontinents, Rodinia, began to
break apart. The continents later recombined to form Pannotia,
600–540 Ma, then finally Pangaea, which
broke apart 180 Ma.
Evolution of life
The general hypothesis is that highly
energetic chemistry produced a self-replicating molecule around 4 billion years ago and half a billion years later the last common ancestor of all life existed. The development of photosynthesis allowed the Sun's energy to be
harvested directly by life forms; the resultant oxygen accumulated in the
atmosphere and formed 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 cellscalled eukaryotes. True
multicellular organisms were formed once the cells within colonies became increasingly specialized. Aided
by the absorption of harmful ultraviolet radiation by
the ozone layer, formerly
ocean-confined life was able to colonize the land surface of Earth.
Since the 1960s, it has been
hypothesized that severe glacial action between 750 and 580 Ma,
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.
Following the Cambrian explosion, about
535 Ma, there have been five major mass extinctions. The most recent such event was 65 Ma, when an asteroid
impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared
some small animals such as mammals, which then resembled shrews. Over the past 65 million
years, mammalian life has diversified, and several million years ago an African
ape-like animal such as Orrorin tugenensis gained
the ability to stand upright. This enabled tool use and encouraged
communication that provided the nutrition and stimulation needed for a larger
brain, which allowed the evolution of the human race. The development of
agriculture, and then civilization, allowed humans to influence the Earth in a
short time span as no other life form had, affecting both the nature and
quantity of other life forms.
The present pattern of ice ages began about 40 Ma and then
intensified during the Pleistocene about 3 Ma. High-latitude regions have since undergone repeated
cycles of glaciation and thaw, repeating every 40–100,000 years. The last
continental glaciation ended 10,000 years ago.
Composition and structure
Earth is a terrestrial planet, meaning
that it is a rocky body, rather than a gas giant likeJupiter. It is the
largest of the four solar terrestrial planets in size and mass. Of these four
planets, Earth also has the highest surface gravity,
the strongest magnetic field, and fastest rotation, and is probably the only one with
active plate tectonics. It also has the highest density in the Solar System at 5.515 g/cm3,
slightly more than Mercury’s density
of 5.427 g/cm3.
Shape
The shape of the Earth approximates an oblate spheroid, a
sphere flattened along the axis from pole to pole such that there is a bulge around the equator. This bulge results from the rotation of the Earth, and causes the diameter
at the equator to be 43 km larger than thepole-to-pole
diameter. The average diameter of
the reference spheroid is about 12,742 km, which is approximately (40,000/π) km, as the meter was originally defined as 1/10,000,000
of the distance from the Equator to the North Pole through Paris, France.
Local topographydeviates
from this idealized spheroid, although on a global scale, these deviations are
small: Earth has a tolerance of
about one part in about 584, or 0.17%, from the reference spheroid, which is
less than the 0.22% tolerance allowed in billiard balls. The
largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level)
and the Mariana Trench (10,911 m below local sea level).
Because of the equatorial bulge, the surface locations farthest from the center
of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.
Chemical composition
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 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.
Nearly 47% of the Earth's crust by
weight consists of oxygen and another 27% of silicon. Other elements, in
descending order, are aluminum (8.1%), iron (5.0%), sodium (2.8%), potassium
(2.6%), and magnesium (2.1%) with all other elements accounting for the
remaining 1.5%. As the table on the right shows, the more common rock
constituents of the Earth's crust are nearly all oxides, with 11 oxides making
up 99% of the weight. 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; silicates make up 95% of the crust by weight.
Chemical composition of the crust
|
|||
Compound
|
Formula
|
Composition
|
|
Continental
|
Oceanic
|
||
SiO2
|
60.2%
|
48.6%
|
|
Al2O3
|
15.2%
|
16.5%
|
|
CaO
|
5.5%
|
12.3%
|
|
MgO
|
3.1%
|
6.8%
|
|
FeO
|
3.8%
|
6.2%
|
|
Na2O
|
3.0%
|
2.6%
|
|
K2O
|
2.8%
|
0.4%
|
|
Fe2O3
|
2.5%
|
2.3%
|
|
H2O
|
1.4%
|
1.1%
|
|
CO2
|
1.2%
|
1.4%
|
|
TiO2
|
0.7%
|
1.4%
|
|
P2O5
|
0.2%
|
0.3%
|
|
Total
|
99.6%
|
99.9%
|
|
Internal structure
The interior of the Earth is divided
into layers, which are defined by their chemical or physical (rheological)
properties. The outer layer of the Earth is a solid crust made of silicate. The
thickness of the crust varies: averaging 6 km under the oceans and 30-50 km
on the continents. Under the crust is a highly viscous solid mantle. Between the crust and
the mantle is a thin layer known as the Mohorovičić discontinuity. The crust and the upper mantle are collectively known as the lithosphere. The
lithosphere rides on top of a portion of the mantle known as the asthenosphere,
which has a lower viscosity than the rest of the mantle. There is a transition zone where
the upper mantle and the mantle meet at between 410 and 660 km. Geologists
believe that matter in the transition zone undergoes significant changes due to
increasing pressure, rearranging its atoms into crystal structures. Beneath the
mantle, an extremely low viscosity liquidouter core surrounds the solid inner core. The inner core may rotate at a
slightly higher rate than the remainder of the planet, advancing by 0.1-0.5°
per year.
Geologic layers of
the Earth
| |||
Earth cutaway
from core to exosphere.
Not to scale.
|
Depth
(km) |
Component Layer
|
Density
(>g/cm3) |
0 - 60
|
Lithosphere
|
-
|
|
0 - 35
|
Crust
|
2.2 - 2.9
|
|
35 - 60
|
Upper mantle
|
3.4 - 4.4
|
|
35 - 2890
|
Mantle
|
3.4 - 5.6
|
|
100 - 700
|
Asthenosphere
|
-
|
|
2890 - 5100
|
Outer core
|
9.9 - 12.2
|
|
5100 - 6378
|
Inner core
|
12.8 - 13.1
|
|
Heat
Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced throughradioactive decay (80%).The major heat-producing
isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the
center of the planet, the temperature may be up to 7,000 K and the
pressure could reach 360 GPa. Because much of the heat is provided
by radioactive decay, scientists surmise that early in Earth history, before
isotopes with short half-lives had been depleted, Earth's heat production would
have been much higher. This extra heat production, twice present-day at approximately
3 billion years ago, would have increased temperature gradients
within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the
production of igneous rocks such as komatiites that are not formed today.
Present-day major heat-producing isotopes
|
||||
Isotope
|
Heat release
W/kg isotope |
Half-life
years |
Mean mantle concentration
kg isotope/kg mantle |
Heat release
W/kg mantle |
238U
|
9.46 × 10−5
|
4.47 × 109
|
30.8 × 10−9
|
2.91 × 10−12
|
235U
|
5.69 × 10−4
|
7.04 × 108
|
0.22 × 10−9
|
1.25 × 10−13
|
232Th
|
2.64 × 10−5
|
1.40 × 1010
|
124 × 10−9
|
3.27 × 10−12
|
40K
|
2.92 × 10−5
|
1.25 × 109
|
36.9 × 10−9
|
1.08 × 10−12
|
Note that the total heat released by
any isotope includes the total energy released in its decay chain; for
example, the U238 entry
includes the contribution from U234 as well.
The mean heat loss from the Earth is 87 mW m−2,
for a global heat loss of 4.42 × 1013 W. 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. More of the heat in the 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 in the oceans because the crust there
is much thinner than that of the continents.
Tectonic plates
-
|
|
Plate
name
|
Area
106 km2
|
103.3
|
|
78.0
|
|
75.9
|
|
67.8
|
|
60.9
|
|
47.2
|
|
43.6
|
|
The mechanically rigid outer layer of
the Earth, the lithosphere, is broken into pieces called tectonic plates. These
plates are rigid segments that move in relation to one another at one of three
types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart,
and Transform boundaries, in which two plates slide past one
another laterally. Earthquakes,
volcanic activity, mountain-building,
and oceanic trench formation can occur along these plate
boundaries.[91] 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, and
their motion is strongly coupled with convection patterns inside the Earth's
mantle.
As the tectonic plates migrate across
the planet, the ocean floor 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 continually recycles the oceanic crust back into the mantle. Because of this
recycling, most of the ocean floor is less than 100 million years in age. The oldest oceanic crust
is located in the Western Pacific, and has an estimated age of about 200 millionyears. By comparison, the oldest dated
continental crust is 4030 million years
old.
The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, theCaribbean Plate,
the Nazca Plate off the west coast of South America and the Scotia Platein the
southern Atlantic Ocean. The
Australian Plate fused with the 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/year and the Pacific Plate moving
52–69 mm/year. At the other extreme, the slowest-moving plate is the
Eurasian Plate, progressing at a typical rate of about 21 mm/year.
Surface
The Earth's terrain varies greatly from place to place.
About 70.8% 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, 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 because 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 coral reefs, and
large meteorite impacts[99] also act to reshape the landscape.
-
Present-day
Earth altimetry and bathymetry.
Data from theNational Geophysical Data Center's TerrainBase Digital Terrain Model.
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. Sedimentary rock is formed from the accumulation of
sediment that becomes compacted together. Nearly 75% of the continental
surfaces are covered by sedimentary rocks, although they form only about 5% of
the crust. 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 the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene andolivine. Common
carbonate minerals include calcite (found in limestone) and dolomite.
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. 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 1.3×107 km2 of cropland and 3.4×107 km2 of pastureland.
The elevation of the land surface of
the Earth varies from the low point of −418 m at the Dead Sea, to a
2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The
mean height of land above sea level is 840 m.
Hydrosphere
-
Elevation histogram
of the surface of the Earth
The abundance of water on Earth's
surface is a unique feature that distinguishes the "Blue Planet" from
others in the Solar System. 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 down to a depth of
2,000 m. The deepest underwater location isChallenger Deep of the Mariana Trench in the Pacific Ocean with a depth of -10,911.4 m.
The mass of the oceans is approximately
1.35×1018 metric
tons, or about 1/4400 of the total mass of the Earth. The oceans
cover an area of 3.618×108 km2 with a mean depth of 3,682 m,
resulting in an estimated volume of 1.332×109 km3. If all the land on Earth were spread
evenly, water would rise to an altitude of more than 2.7 km. About 97.5% of the water is saline,
while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is
currently ice.
The average salinity of the Earth's oceans is about
35 grams of salt per kilogram of sea water (35 ‰). Most of this
salt was released from volcanic activity or extracted from cool, igneous rocks. The oceans are also a reservoir of
dissolved atmospheric gases, which are essential for the survival of many
aquatic life forms. Sea water has an important influence on the world's
climate, with the oceans acting as a large heat reservoir. Shifts in the oceanic temperature
distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.
Atmosphere
The atmospheric pressure on
the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km. It is 78%
nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and
other gaseous molecules. The height of the troposphere varies with latitude, ranging between
8 km at the poles to 17 km at the equator, with some variation
resulting from weather and seasonal factors.
Earth's biosphere has significantly
altered its atmosphere. Oxygenic
photosynthesis evolved 2.7 billion years ago, forming the
primarily nitrogen-oxygen atmosphere of today. This change enabled the
proliferation of aerobic organisms as well as the formation of the ozone
layer which blocks ultraviolet solar radiation,
permitting life on land. Other atmospheric functions important to life on Earth
include transporting water vapor, providing useful gases, causing small meteors to
burn up before they strike the surface, and moderating temperature. 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 and ozone are the primary greenhouse gases in the Earth's atmosphere. Without
this heat-retention effect, the average surface would be −18 °C, in
contrast to the current +15 °C, and life would likely not exist.
Weather and climate
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. 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 heat energy.
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°.Ocean currents are also important factors in determining climate,
particularly the thermohaline circulation that
distributes heat energy from the equatorial oceans to the polar regions.
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 settles to the surface as precipitation.[116] 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, topological features and temperature
differences determine the average precipitation that falls in each region.
The amount of solar energy reaching the
Earth's decreases with increasing latitude. At higher latitudes the sunlight
reaches the surface at a 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 per per degree of latitude away from the equator. The Earth can be sub-divided 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.[121] Climate can also be classified based
on the temperature and precipitation, with the climate regions characterized by
fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's
student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continentaland cold polar), which are further divided into
more specific subtypes.
Upper atmosphere
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This view from orbit
shows the full Moon partially obscured
and deformed by the
Earth's atmosphere. NASA image.
Above the troposphere, the atmosphere
is usually divided into the stratosphere, mesosphere, and thermosphere. 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 Earth's magnetic fields interact with the solar wind.[122] 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 above the Earth's surface, is a working definition for
the boundary between atmosphere and space.
Thermal energy causes some of the
molecules at the outer edge of the Earth's atmosphere have their velocity
increased to the point where they can escape from the planet's gravity. This
results in a slow but steady leakage of the atmosphere into space. Because unfixedhydrogen has a low molecular weight, it can
achieve escape velocity more readily and it leaks into outer
space at a greater rate than other gasses. The
leakage of hydrogen into space contributes to the pushing of the Earth 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 presumed to
have been a necessary precondition for the widespread accumulation of oxygen in
the atmosphere. Hence the ability of hydrogen to escape from the Earth's
atmosphere may have influenced the nature of life that developed on the planet. 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.
Magnetic field
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Schematic of Earth's
magnetosphere. The solar wind
flows from left to right
The Earth's magnetic field is
shaped roughly as a magnetic dipole,
with the poles currently located proximate to the planet's geographic poles. At
the equator of the magnetic field, the magnetic field strength at the planet's
surface is 3.05
× 10−5 T,
with global magnetic dipole moment of 7.91 × 1015 T m3.
According to dynamo theory, the
field is generated within the molten outer core region where heat creates
convection motions of conducting materials, generating electric currents. These
in turn produce the Earth's magnetic field. The convection movements in the
core are chaotic; the magnetic poles drift and periodically change alignment.
This results in field reversals at
irregular intervals averaging a few times every million years. The most recent
reversal occurred approximately 700,000 years ago.
The field forms the magnetosphere,
which deflects particles in the solar wind. The
sunward edge of the bow shock is located at about 13 times the
radius of the Earth. The collision between the magnetic field and the solar
wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles.
When the plasma enters the Earth's atmosphere at the
magnetic poles, it forms the aurora.
Future of the Earth
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The life cycle of the
Sun
The future of the planet is closely
tied to that of the Sun. As a result of the steady accumulation of helium at
the Sun's core, the star's total luminosity will slowly increase. The luminosity
of the Sun will grow by 10% over the next 1.1 Gyr (1.1 billion years) and by 40%
over the next 3.5 Gyr. Climate models indicate that the rise in radiation
reaching the Earth is likely to have dire consequences, including the loss of
the planet's oceans.
The Earth's increasing surface
temperature will accelerate the inorganic CO2 cycle, reducing its
concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 500 million to 900 million years.
The lack of vegetation will result in the loss of oxygen in the atmosphere, so
animal life will become extinct within several million more years. After another billion years all
surface water will have disappeared and the mean global temperature will reach
70 °C (158 °F). The Earth is expected to be effectively habitable for
about another 500
million years from that
point, although this may be
extended up to 2.3 billion years if
the nitrogen is removed from the atmosphere. Even if the Sun were eternal and
stable, the continued internal cooling of the Earth would result in a loss of
much of its CO2 due to
reduced volcanism, and 35%
of the water in the oceans would descend to the mantle due to reduced steam venting from
mid-ocean ridges.
The Sun, as part of its evolution, will
become a red giant in about 5 Gyr. Models predict that
the Sun will expand out to about 250 times its present radius, roughly
1 AU (150,000,000 km). Earth's fate is less clear. As a red giant, the Sun
will lose roughly 30% of its mass, so, without tidal effects, the Earth will
move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star
reaches it maximum radius. The planet was therefore initially expected to
escape envelopment by the expanded Sun's sparse outer atmosphere, though most,
if not all, remaining life would have been destroyed by the Sun's increased
luminosity (peaking at about 5000 times its present level). A 2008 simulation
indicates that Earth's orbit will decay due to tidal effects and
drag, causing it to enter the red giant Sun's atmosphere and be vaporized.
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