Earth

  The Earth is the third planet from the Sun, in our Solar System. Its is the only planet where life is known. Its third among the terrestrial planets and also the biggest. Its referred to as the World. Up, form above the atmosphere, the Earth looks blue in colour. Its because of the large amounts of water in it. The Earth, though a rocky planet, is mostly filled with water, i.e 70%(approx.) and the land cover only about 30% of the area of Earth.

  The Earth is said to have formed 4.5 billion years ago due to the condensation of the Molecular cloud which also led to the formation of the our Solar System including the Sun. Earth has more than a 1000 species of living beings and so much more have to be discovered. 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 its geological history and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life for at least another 500 million years.


Characteristics: 

1. Rotational period: 23 hours 56 minutes 9 seconds
2. Revolution period: 365 days 6 hours.
3. Aphelion: 15,20,98,232 km
4. Perihelion: 14,70,98,290 km
5. Speed: 29.78 m/s(1,07, 200 km/h)
6. Radius: 6371 km
7. Polar radius: 635.8 km
8. Equatorial radius: 6378.1 km
9. Surface Area: 510,072,000 km²
10. Escape Velocity: 11.186 km/s
11. Volume: 1.08321×10¹² km³
12. Mass: 5.9736×10²⁴ kg
13. Acceleration due to gravity(g): 9.780327 m/s²

• Surface pressure : 101.325 kPa (MSL or Mean Sea Level)
• Composition : 
•       78.08% nitrogen (N₂)       
•       20.95% oxygen (O₂)
•       0.93% argon
•       0.038% carbon dioxide               
•       About 1% water vapor (varies with climate)
                           
  

Geology: Earth's outer surface 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 with salt water oceans, the remainder consisting of continents and islands which together have many lakes and other sources of water contributing to the hydrosphere. Liquid water, necessary for all known life, is not known to exist in equilibrium on any other planet's surface. 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.

Internal Structure: The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, its core is distinct. The outer layer of the Earth is a chemically distinct silicate solid crust or sand which is mainly made up of silica, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovicic discontinuity(Mohorovicic - an Arabian scientist who found a discontinuity between the two layers), and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents. The temperature rises with an average of 1°C per kilometer.

Layers of the Earth

 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 comprised. 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 kilometers 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.

Crust: The outer most layer, the crust, is categorized into two parts, the Oceanic crust and the continental crust. The Oceanic crust is the smallest part of Earth, only 0.099% of its mass and reaching a small depth of 0-6 miles (0-10 kilometers). In the beginning of time, it was possible that this area did not exist for through frequent volcanic activity does only the crust form. Evidence of this is marked by the oceanic ridge system, which is a 25,000 mile (40,000-kilometer) array of many volcanoes which creates layer after layer of new crust at the rate of 17 km3 per year. The ocean floor is covered in basalt originating from volcanic activity and as a matter of fact, Iceland and Hawaii are two island systems that emerged from the accumulated basalt. 


Continental crust: The second smallest area of the Earth is the Continental crust, making up only 0.374% of the Earth's mass and extending a short depth of 0 - 31 miles (0-50 kilometers). Looking at the percent by composition, the continental crust makes up only 0.554% of the mantle-crust mass. The layer is composed primarily of crystalline rocks made of low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates). This is the outer part of the Earth composed essentially of crystalline rocks. The continental crust and the oceanic crust are also referred to as the lithosphere because of the cool and rocky conditions that exist in its chemical composition.

Crust

The next layer, the Transition region comprises 7.5% of Earth's mass with a depth of 250-406 miles (400-650 kilometers). This layer is also known as the mesosphere and is 11.1% of the mantle-crust. It is made of mainly basaltic magmas with amounts of calcium, aluminum and garnet (an aluminum-bearing silicate mineral). The layer becomes dense when the garnet mineral cools but is buoyant and light when subject to heat due to the low melting points

The D" layer of Earth is about 3% of Earth's mass, is 125 to 188 miles (200 to 300 kilometers) thick and covers about 4% of the mantle-crust mass. This layer, in terms of whether it is part of the lower mantle or an independent layer is still somewhat unclear. Based on evidence collected from seismic discontinuities, the D" layer might differ in chemical composition from the lower mantle above it.

Mantle: Lower down, we encounter the upper mantle. Through excavations in volcanoes, scientists have found that this part of the crust composes of 15.3% of the total mantle-crust mass and is made of crystalline forms of Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3. The upper mantle makes up 10.3% of the Earth's mass, extending a depth of 6-250 miles (10-400 kilometers). A relatively large portion when compared to the other interior layers. This layer is not completely made of solid minerals for scientists speculate that the asthenosphere could be partly liquid molten.

Looking at the lower mantle, its chemical omposition includes silicon, magnesium, and oxygen. Most likely, it probably also contains some iron, calcium, and aluminum. This layer is comprised of 72.9% of the antle-crust mass, making the Earth abundant in the chemical elements of silicon, magnesium and oxygen, the layer's primary components.

Mantle

Core: The outer core is in the range of 200 to 300 kilometers (125 to 188 miles) thick and represents about 4% of the mantle-crust mass. This layer is sometimes identified as part of the lower mantle due to its geographical nature. However, studies on seismic discontinuities suggest that this "D" layer might differ chemically rom the lower mantle lying above it.