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Earth
planet, one of nine planets in the
solar system, the only planet known
to harbor life, and the home of
human beings. From space Earth resembles
a big blue marble with swirling
white clouds floating above blue
oceans. About 71 percent of Earth
surface is covered by water, which
is essential to life. The rest is
land, mostly in the form of continents
that rise above the oceans.
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History
of Earth: Based on the available evidence,
current scientists have been able to reconstruct
detailed information about the planet's
past. Earth formed 4.567 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.
Unlike
the other planets, Earth has a unique
set of characteristics ideally suited
to supporting life as we know it. It
is neither too hot, like Mercury, the
closest planet to the Sun, nor too cold,
like distant Mars and the even more
distant outer planets Jupiter, Saturn,
Uranus, Neptune, and tiny Pluto. Earth
atmosphere includes just the right amount
of gases that trap heat from the Sun,
resulting in a moderate climate suitable
for water to exist in liquid form. The
atmosphere also helps block radiation
from the Sun that would be harmful to
life. Earth atmosphere distinguishes
it from the planet Venus, which is otherwise
much like Earth. Venus is about the
same size and mass as Earth and is also
neither too near nor too far from the
Sun. But because Venus has too much
heat-trapping carbon dioxide in its
atmosphere, its surface is extremely
hot 462°C (864°F) hot enough
to melt lead and too hot for life to
exist.
Although
Earth is the only planet known to have
life, scientists do not rule out the
possibility that life may once have
existed on other planets or their moons,
or may exist today in primitive form.
Mars, for example, has many features
that resemble river channels, indicating
that liquid water once flowed on its
surface. If so, life may also have evolved
there, and evidence for it may one day
be found in fossil form. Water still
exists on Mars, but it is frozen in
polar ice caps, in permafrost, and possibly
in rocks below the surface.
For thousands of years, human beings
could only wonder about Earth and the
other observable planets in the solar
system. Many early ideasfor example,
that the Earth was a sphere and that
it traveled around the Sunwere based
on brilliant reasoning. However, it
was only with the development of the
scientific method and scientific instruments,
especially in the 18th and 19th centuries,
that humans began to gather data that
could be used to verify theories about
Earth and the rest of the solar system.
By studying fossils found in rock layers,
for example, scientists realized that
the Earth was much older than previously
believed. And with the use of telescopes,
new planets such as Uranus, Neptune,
and Pluto were discovered.
In
the second half of the 20th century,
more advances in the study of Earth
and the solar system occurred due to
the development of rockets that could
send spacecraft beyond Earth. Human
beings were able to study and observe
Earth from space with satellites equipped
with scientific instruments. Astronauts
landed on the Moon and gathered ancient
rocks that revealed much about the early
solar system. During this remarkable
advancement in human history, humans
also sent unmanned spacecraft to the
other planets and their moons. Spacecraft
have now visited all of the planets
except Pluto. The study of other planets
and moons has provided new insights
about Earth, just as the study of the
Sun and other stars like it has helped
shape new theories about how Earth and
the rest of the solar system formed.
As
a result of this recent space exploration,
we now know that Earth is one of the
most geologically active of all the
planets and moons in the solar system.
Earth is constantly changing. Over long
periods of time land is built up and
worn away, oceans are formed and re-formed,
and continents move around, break up,
and merge.
Life
itself contributes to changes on Earth,
especially in the way living things
can alter Earth's atmosphere. For example,
Earth at one time had the same amount
of carbon dioxide in its atmosphere
as Venus now has, but early forms of
life helped remove this carbon dioxide
over millions of years. These life forms
also added oxygen to Earth atmosphere
and made it possible for animal life
to evolve on land.
A
variety of scientific fields have broadened
our knowledge about Earth, including
biogeography, climatology, geology,
geophysics, hydrology, meteorology,
oceanography, and zoogeography. Collectively,
these fields are known as Earth science.
By studying Earth's atmosphere, its
surface, and its interior and by studying
the Sun and the rest of the solar system,
scientists have learned much about how
Earth came into existence, how it changed,
and why it continues to change.
Earth
is the third planet from the Sun, after
Mercury and Venus. The average distance
between Earth and the Sun is 150 million
km (93 million mi). Earth and all the
other planets in the solar system revolve,
or orbit, around the Sun due to the
force of gravitation. The Earth travels
at a velocity of about 107,000 km/h
(about 67,000 mph) as it orbits the
Sun. All but one of the planets orbit
the Sun in the same planethat is, if
an imaginary line were extended from
the center of the Sun to the outer regions
of the solar system, the orbital paths
of the planets would intersect that
line. The exception is Pluto, which
has an eccentric orbit.
Earth
orbital path is not quite a perfect
circle but instead is slightly elliptical
(oval-shaped). For example, at maximum
distance Earth is about 152 million
km (about 95 million mi) from the Sun;
at minimum distance Earth is about 147
million km (about 91 million mi) from
the Sun. If Earth orbited the Sun in
a perfect circle, it would always be
the same distance from the Sun.
The
solar system, in turn, is part of the
Milky Way Galaxy, a collection of billions
of stars bound together by gravity.
The Milky Way has armlike discs of stars
that spiral out from its center. The
solar system is located in one of these
spiral arms, known as the Orion arm,
which is about two-thirds of the way
from the center of the Galaxy. In most
parts of the Northern Hemisphere, this
disc of stars is visible on a summer
night as a dense band of light known
as the Milky Way.
Earth
is the fifth largest planet in the solar
system. Its diameter, measured around
the equator, is 12,756 km (7,926 mi).
Earth is not a perfect sphere but is
slightly flattened at the poles. Its
polar diameter, measured from the North
Pole to the South Pole, is somewhat
less than the equatorial diameter because
of this flattening. Although Earth is
the largest of the four planetsMercury,
Venus, Earth, and Marsthat make up the
inner solar system (the planets closest
to the Sun), it is small compared with
the giant planets of the outer solar
systemJupiter, Saturn, Uranus, and Neptune.
For example, the largest planet, Jupiter,
has a diameter at its equator of 143,000
km (89,000 mi), 11 times greater than
that of Earth. A famous atmospheric
feature on Jupiter, the Great Red Spot,
is so large that three Earths would
fit inside it.
Earth
has one natural satellite, the Moon.
The Moon orbits the Earth, completing
one revolution in an elliptical path
in 27 days 7 hr 43 min 11.5 sec. The
Moon orbits the Earth because of the
force of Earth gravity. However, the
Moon also exerts a gravitational force
on the Earth. Evidence for the Moon
gravitational influence can be seen
in the ocean tides. A popular theory
suggests that the Moon split off from
Earth more than 4 billion years ago
when a large meteorite or small planet
struck the Earth.
As
Earth revolves around the Sun, it rotates,
or spins, on its axis, an imaginary
line that runs between the North and
South poles. The period of one complete
rotation is defined as a day and takes
23 hr 56 min 4.1 sec. The period of
one revolution around the Sun is defined
as a year, or 365.2422 solar days, or
365 days 5 hr 48 min 46 sec. Earth also
moves along with the Milky Way Galaxy
as the Galaxy rotates and moves through
space. It takes more than 200 million
years for the stars in the Milky Way
to complete one revolution around the
Galaxy center.
Earth
axis of rotation is inclined (tilted)
23.5° relative to its plane
of revolution around the Sun. This inclination
of the axis creates the seasons and
causes the height of the Sun in the
sky at noon to increase and decrease
as the seasons change. The Northern
Hemisphere receives the most energy
from the Sun when it is tilted toward
the Sun. This orientation corresponds
to summer in the Northern Hemisphere
and winter in the Southern Hemisphere.
The Southern Hemisphere receives maximum
energy when it is tilted toward the
Sun, corresponding to summer in the
Southern Hemisphere and winter in the
Northern Hemisphere. Fall and spring
occur in between these orientations.
The
atmosphere is a layer of different gases
that extends from Earth surface to the
exosphere, the outer limit of the atmosphere,
about 9,600 km (6,000 mi) above the
surface. Near Earth surface, the atmosphere
consists almost entirely of nitrogen
(78 percent) and oxygen (21 percent).
The remaining 1 percent of atmospheric
gases consists of argon (0.9 percent);
carbon dioxide (0.03 percent); varying
amounts of water vapor; and trace amounts
of hydrogen, nitrous oxide, ozone, methane,
carbon monoxide, helium, neon, krypton,
and xenon.
The
layers of the atmosphere are the troposphere,
the stratosphere, the mesosphere, the
thermosphere, and the exosphere. The
troposphere is the layer in which weather
occurs and extends from the surface
to about 16 km (about 10 mi) above sea
level at the equator. Above the troposphere
is the stratosphere, which has an upper
boundary of about 50 km (about 30 mi)
above sea level. The layer from 50 to
90 km (30 to 60 mi) is called the mesosphere.
At an altitude of about 90 km, temperatures
begin to rise. The layer that begins
at this altitude is called the thermosphere
because of the high temperatures that
can be reached in this layer (about
1200°C, or about 2200°F).
The region beyond the thermosphere is
called the exosphere. The thermosphere
and the exosphere overlap with another
region of the atmosphere known as the
ionosphere, a layer or layers of ionized
air extending from almost 60 km (about
50 mi) above Earth surface to altitudes
of 1,000 km (600 mi) and more.
Earth
atmosphere and the way it interacts
with the oceans and radiation from the
Sun are responsible for the planet climate
and weather. The atmosphere plays a
key role in supporting life. Almost
all life on Earth uses atmospheric oxygen
for energy in a process known as cellular
respiration, which is essential to life.
The atmosphere also helps moderate Earth
climate by trapping radiation from the
Sun that is reflected from Earth surface.
Water vapor, carbon dioxide, methane,
and nitrous oxide in the atmosphere
act as â œgreenhouse gases.
Like the glass in a greenhouse, they
trap infrared, or heat, radiation from
the Sun in the lower atmosphere and
thereby help warm Earth surface. Without
this greenhouse effect, heat radiation
would escape into space, and Earth would
be too cold to support most forms of
life.
Other
gases in the atmosphere are also essential
to life. The trace amount of ozone found
in Earth stratosphere blocks harmful
ultraviolet radiation from the Sun.
Without the ozone layer, life as we
know it could not survive on land. Earth
atmosphere is also an important part
of a phenomenon known as the water cycle
or the hydrologic cycle.
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