Earth
Earth is the the third planet from the Sun, the densest planet in the Solar System, largest of the Solar System's four terrestrial planets, and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed about 4.54billion years ago.
Geological history
Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, protoplanets, and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Gya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.
Internal structure
Earth's
interior, like that of the other terrestrial planets, is divided into layers by
their chemical or physical (rheological) properties, but unlike the other
terrestrial planets, it has a distinct outer and inner core. The outer layer is
a chemically distinct silicate solid crust, which is underlain by a highly
viscous solid mantle. The crust is separated from the mantle by the Mohorovičić
discontinuity, and the thickness of the crust varies: averaging 6 km (kilometers)
under the oceans and 30–50 km on the continents. The crust and the cold,
rigid, top of the upper mantle are collectively known as the lithosphere, and
it is of the lithosphere that the tectonic plates are composed. Beneath the
lithosphere is the asthenosphere, a relatively low-viscosity layer on which the
lithosphere rides. Important changes in crystal structure within the mantle
occur at 410 and 660 km below the surface, spanning a transition zone that
separates the upper and lower mantle. Beneath the mantle, an extremely low
viscosity liquid outer core lies above a solid inner core. The inner core may
rotate at a slightly higher angular velocity than the remainder of the planet,
advancing by 0.1–0.5° per year. The radius of the inner core is about one fifth
of that of Earth.
Atmosphere
The atmospheric pressure on Earth's
surface averages 101.325 kPa, with a scale height of about 8.5 km. It
has a composition of 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 Gya, forming the primarily nitrogen–oxygen atmosphere of
today.This change enabled the proliferation of aerobic organisms and,
indirectly, the formation of the ozone layer due to the subsequent conversion
of atmospheric O2 into O3. The ozone layer blocks
ultraviolet solar radiation, permitting life on land.Other atmospheric
functions important to life 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 atmosphere.
Without this heat-retention effect, the average surface temperature would be
−18 °C, in contrast to the current +15 °C, and life would likely not
exist.
Magnetic field
The main part of Earth's magnetic field is
generated in the core, the site of a dynamo process that converts kinetic
energy of fluid convective motion into electrical and magnetic field energy.
The field extends outwards from the core, through the mantle, and up to Earth's
surface, where it is, to rough approximation, a dipole. The poles of the dipole
are located close to Earth's geographic poles. At the equator of the magnetic
field, the magnetic-field strength at the surface is 3.05 × 10−5 T,
with global magnetic dipole moment of 7.91 × 1015 T m3.The
convection movements in the core are chaotic; the magnetic poles drift and
periodically change alignment. This causes field reversals at irregular
intervals averaging a few times every million years. The most recent reversal
occurred approximately 700,000 years ago.Rotation
Earth's
rotation period relative to the Sun—its mean solar day—is 86,400 seconds
of mean solar time (86,400.0025 SI seconds). Because Earth's solar
day is now slightly longer than it was during the 19th century due to tidal
deceleration, each day varies between 0 and 2 SI ms longer.
Earth's
rotation period relative to the fixed stars, called its stellar day by
the International Earth Rotation and Reference Systems Service (IERS), is
86,164.098903691 seconds of mean solar time (UT1), or 23h 56m
4.098903691s.Earth's rotation period relative to the precessing or
moving mean vernal equinox, misnamed its sidereal day, is
86,164.09053083288 seconds of mean solar time (UT1) (23h 56m
4.09053083288s) as of 1982. Thus the sidereal day is shorter than
the stellar day by about 8.4 ms.The length of the mean solar day in SI
seconds is available from the IERS for the periods 1623–2005 and
1962–2005
