EARTH : a living and capricious planet

 A violent past:

    At first, Earth was a hot planet with a molten surface. Debris from the solar nebula in the form of meteorites and comets continuously crashed onto the young planet's glowing surface. These heavy bombardments have helped increase the temperature of the Earth. Then, shortly after its formation, about 4.6 billion years ago, the Earth was struck by an object the size of Mars. The impact produced considerable heat. Debris from the explosion caused by this collision was thrown into space and coalesced to form the Moon. Slowly the temperature of the Earth began to drop. Thus, a planet with a solid surface was formed, with oceans, continents and an atmosphere. Indeed, the Earth has been the bearer of life since its youth. And now she is approaching middle age. It is estimated that the Sun will continue to bathe it in heat, which generates life, for almost 5 billion years.

The transformation of the Earth:

    Molten earthly materials began to separate into layers over 4 billion years ago. Heavy materials, rich in iron, formed the glowing core at the center of the planet. Silica and the lighter elements floating on the surface made up the crust. Between the core and the crust, materials rich in silica and magnesium, strong but deformable, have become what is called the mantle. On the surface, light granite rocks thickened the crust, forming the first continents.

Eventful past!

    On this picture, an imaginary representation of the wild landscape of young Earth. The fragile crust was undoubtedly damaged by debris from space and lava flows. Meteorites crashing into Earth formed huge craters and sank into the hot mantle, sending gigantic streams of molten rock to the surface.
    Little by little, the thin crust thickened, but sometimes pieces of cooled crust plunged back into the molten mantle, to be melted again.

End of the dinosaurs!

     For over 1000 million years, dinosaurs ruled the earth, but they suddenly disappeared 65   million years ago. Their extinction was probably caused by the collision of a huge meteorite or   comet with the Earth. The impact must then have plunged the world into a cloud of dust that   blocked the Sun for several months. In this frozen darkness, most of the plants and dinosaurs   perished. However, some small hibernating mammals, such as raccoons, were able to survive.

The oceans :

     The water in the oceans could come from comets that have crashed on Earth. 
 >> A comet is a huge ball of ice and rock.
     The water could also have been generated by water vapor produced by molten rocks   (magma) flowing on the surface of the Earth. By condensing, this vapor would have formed   clouds in the atmosphere before falling back to Earth in the form of rain.

The atmosphere:

    Young Earth's atmosphere was rich in volcanic gases such as carbon dioxide.

 It is believed that this change was caused by tiny, primitive living organisms that were releasing oxygen. These tufts of vegetation , made up of microorganisms called cyanobacteria, capture sunlight for food and release oxygen.

The sun :

    Our sun is a medium-sized star, identical to billions of others in the galaxy. Without its heat, the Earth would be uninhabitable. Scientists have calculated that the sun will shine for another 5 billion years before it depletes its hydrogen. Then, it will undergo an expansion, reaching 1000 times its size, and will become an incandescent sphere called a red giant which will destroy the Earth.

Ice ages :

    The Earth has, however, been partially covered with ice during its history. When the climate cooled, the ice spread from the poles towards the equator, then retreated when the planet  warmed.
    This warming may be due to a slow oscillation of the Earth's axis which has altered its distance from the Sun. Today's glaciers allow us to imagine what the Earth looked like at the time.