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The Earth

Our planet Earth was formed at about 4.54 billion years ago. It is a special planet as life appeared a billion years later on its surface and we live on it. Earth is the third planet from the Sun and the fifth-largest of the eight planets in the Solar System. It is a terrestrial planet, i.e. a rocky body. It is the largest in size of the four solar terrestrial planets, i.e. planets that are solid, made of rocks, namely Mercury, Venus, Earth and Mars in radius (average 6,371 km) surface area (510,072,000 km2) and mass (5.9736×1024 kg), the highest density (mean density 5.515 g/cm³), the highest surface gravity (9.780327 m/s2), the strongest magnetic field (surface at the equator: 3.05 × 10?5 T ) and the fastest rotation  (86,400.0025 seconds). The mass of the Earth is composed 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% all other elements.

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. Unlike other planets, Earth has a distinct outer and inner core. The outer core is a liquid layer about 2,266 kilometers thick composed of iron and nickel while the inner core is a primarily solid sphere about 1,216 km and it is believed to consist of an iron-nickel alloy.

The events that have occurred during the history of the Earth are being described by the geological time scale. Eons are divided into eras, which are in turn divided into periods, epochs and ages.  Each era on the scale is separated from the next by a major event or change. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Tertiary extinction event, which marked the demise of the dinosaurs.

The climate has been changing in non-periodic ways over most scales of time, that is marked by colder temperatures (glacial periods) and periods of warmer climate within an ice age (interglacial periods). The last glacial period was the most recent glacial period within the current ice age, occurring in the Pleistocene epoch, which began about 70,000 and ended about 15,000 years ago. The current Holocene epoch is the interglacial we are presently in.

The crust is composed of a variety of igneous, metamorphic, and sedimentary rocks. The crust is underlain by the mantle. It differs from the crust in mechanical characteristics and chemical composition. The distinction between crust and mantle is based on chemistry, rock types, rheology and seismic characteristics. 

The upper mantle has two main zones: the inner asthenosphere, 200 km thick, which is composed of flowing rock in state of plasticity and the about 50 to 120 km rigid rock lithosphere above, which is divided in tectonic plates. There are presently seven major and many minor plates that change shape and position with time as they interact to eachother and the surface undergoes reshaping over geological time periods because of tectonics and erosion that contributes substantially at times.

The atmosphere extends outwards in the magnetosphere and has no boundary. Most of the atmosphere's mass is below 10 km from sea level. Troposphere is called the lowest layer. When a part of it is heated by the Sun and it expands and then rises, and it is replaced by cooler, higher density air. As a result of heat an atmospheric circulation starts. This type of circulations drive the weather and in the long run the climate through redistribution of heat energy.

Earth is not isolated in the Cosmos and interacts gravitationally with all nearby massive objects, the Sun and the Moon been most important in this due to mass and poximity. Tides are important for both the Earth and the Moon motion and rotation. The Moon orbits the Earth for 4.53 billion years and induces ocean tides, it stabilizes the axis of our planet and slows down the planet's rotation.

Milankovitch put forward a theory that ice ages are caused to changes of insolation conditions of our planet especially at high  latitudes, due to variations of Earth's orbit and orientation of axis due to precession, change of obliquity and eccentricity of the orbit. The Moon stabilizes Earth’s axis. If the Moon were not present, the torques exerted on the Earth would be smaller, and large variations in obliquity resulting from its chaotic behaviour would have caused dramatic obliquity and climate changes. The Moon is a potential climate regulator as it stabilizes the axis of the Earth.

The magnetic field of the Earth is dipolar to first approximation, changes slightly with time (secular variation) and some times there are sudden changes and the position of the magnetic poles and even magnetic polarity changes dramatically over periods of some 100000 years.

Reversals last several thousand years and the magnetic field does not vanish but becomes very complicated, as the solar magnetic field during solar maximum.

The magnetic field of the Earth is represented by spherical harmonics using some 200 terms that are time varying. The International Geomagnetic Reference Field, IGRF, is a standard model used for geomagnetic filed and has coefficients that changes every 5 years.

The magnetic field of the Earth interacts with the solar wind (see the Sun) and creates the magnetosphere, which is a very important component that acts like a shield of our planet, as it helps to  keep the atmosphere, protecting us from the solar wind that otherwise could have replenished the atmosphere and let many more cosmic rays to arrive at the ground.

Several Earth observating missions by the European scientific community have been held.  ESA has the most important role in the field counting a great number of missions enhancing the European contribution. 

Historical missions: Landsat MSS/RVB, IRS-P3, Jers 1 and Nimbus-7.
Current Missions of ESA, briefly: Proba, ALOS, FORMOSAT-2, GOSAT, Ikonos-2, IRS-P6, KOMPSAT-1, KOMPSAT-2, Landsat TM/ETM, NOAA AVHRR, Odin, QuickSCAT, SciSat, SeaWiFS, SPOT-4, Terra/ Aqua  MODIS, UK-DMC.

LINKS:

http://www.esa.int/esaCP/index.html
http://www.nasa.gov/missions/index.html
http://www.eduspace.esa.int/common/links.asp
http://earthobservatory.nasa.gov
http://www.eumetsat.int/Home/index.htm

PICTURES:

Meteosat-2 artificially-coloured visible channel full-disc Earth image. Developed by ESA and since 1995 operated by EUMETSAT

From http://pds.nasa.gov/planets/special/earth.htm

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