Earth

(TFS) - Earth is the third planet from the Solar system and the fifth largest.

The earth orbits itself in 23h 56min 4s and turns around the sun in 365 days at a speed of 108 000 km / h. The rotation of the Earth upon itself leads to the alternation of day and night. Its movement around the sun engenders the different seasons. The surface of the earth is 71% covered by oceans.

The earth has only one satellite: The Moon.

Planet Profile

orbit: 149,600,000 km (1.00 AU) from Sun

diameter: 12,756.3 km

mass: 5.972e24 kg

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.

Composition

The Earth's crust is about 30 to 50 km thick on average and 5 to 10 km at the ocean level. This part is rigid similiarly to the upper mantle while the whole floats on the lower mantle which is softer. The nucleus consists of an internal solid nucleus where the temperature varies between is 3 000 to 4 000 °C and an external solid core.

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

0- 40 Crust

40- 400 Upper mantle

400- 650 Transition region

650-2700 Lower mantle

2700-2890 D'' layer

2890-5150 Outer core

5150-6378 Inner core

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms):

atmosphere = 0.0000051

oceans = 0.0014

crust = 0.026

mantle = 4.043

outer core = 1.835

inner core = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:

 Earth-South America

34.6% Iron

29.5% Oxygen

15.2% Silicon

12.7% Magnesium

2.4% Nickel

1.9% Sulfur

0.05% Titanium

The Earth is the densest major body in the solar system.

Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle.

71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface.

Water on Earth

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants.

The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.

Earth's Satellite

Earth has only one natural satellite, the Moon. But Thousands of small artificial satellites have also been placed in orbit around the Earth.

Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they're not really moons, the term "companion" is being used. It is somewhat similar to the situation with Saturn's moons Janus and Epimetheus.

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source: http://nineplanets.org/earth.html