Earth

Earth

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Earth, our home, is the third planet from the sun. It is the densest and fifth-largest of the eight planets in the Solar System. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and, of course, life.


 Earth is the fifth largest of the planets in the solar system — smaller than the four gas giants, Jupiter, Saturn, Uranus andNeptune, but larger than the three other rocky planets,

Mercury, Mars andVenus formation of earth : Scientists think Earth was formed at roughly the same time as the sun and other planets some 4.6 billion years ago, when the solar system coalesced from a giant, rotating cloud of gas and dust known as the solar nebula. 

Earth started off as a waterless mass of rock. Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth's interior, causing some chemicals to rise

to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape. 

Earth's atmosphere and temparature : Earth's atmosphere is roughly 78 percent nitrogen, 21 percent oxygen, with trace amounts of water, argon, carbon dioxide

and other gases. Nowhere else in the solar system can one find an atmosphere loaded with free oxygen, which ultimately proved vital to one of the other unique features of Earth Water vapor, carbon dioxide and other gases in the atmosphere trap heat from the sun, warming Earth. Without this so-called "greenhouse effect," Earth would probably be too cold for life to exist, although a runaway greenhouse effect led to the hellish conditions now seen on Venus.

 The average temperature on Earth is about 61 degrees F (16 C).

But temperatures vary greatly around the world depending on the time of year, ocean and wind currents and weather conditions. Summers tend to be warmer and winters colder. Also, temperatures tend to be higher near the equator and lower near the poles. structure of earth Earth has multiple layers. The ocean basins and the continents compose the crust, the outermost layer. Earth's crust is between three and 46 miles (five and 75 km) deep.

 The thickest parts are under the continents and

the thinnest parts are under the oceans. Earth's crust is made up of several elements: iron, 32 percent; oxygen, 30 percent; silicon, 15 percent; magnesium, 14 percent; sulfur, 3 percent; nickel, 2 percent; and trace amounts of calcium, aluminum and other elements.The crust is divided into huge plates that float on the mantle, the next layer. The plates are constantly in motion; they move at about the same rate as fingernails grow.

 Earthquakes occur when these plates grind against each other. Mountains form when the plates collide and deep trenches form when one plate slides under another

plate. Plate tectonics is the theory explaining the motion of these plates. At the center of the Earth is the core, which has two parts. The solid, inner core of iron has a radius of about 760 miles (about 1,220 km). It is surrounded by a liquid, outer core composed of a nickel-iron alloy. It is about 1,355 miles (2,180 km) thick.

 The inner core spins at a different speed than the rest of the planet. This is

thought to cause Earth's magnetic field. When charged particles from the solar wind collide with air molecules above Earth's magnetic poles, it causes the air molecules to glow, causing the auroras — the northern and southern lights. Earth's moon: Earth's moon is 2,159 miles (3,474 km) wide, about one-fourth of Earth's diameter.

 Earth has one moon, while Mercury and Venus have none and all the other planets in our solar system have two or more.

lifeform on earth : Earth is the only planet in the universe known to possess life. There are several million known species of life, ranging from the bottom of the deepest ocean to a few miles into the atmosphere, and scientists think far more remain to be discovered.

 Scientists figure there are between 5 million and 100 million species on

Earth, but science has only identified about 2 million of them .