Astronauts always speak of the Earth as "The Blue Planet" due to its color.
And the photos captured from space prove it. Those responsible for having these blue tones are the oceans and the gases of the atmosphere, that is, the two components external to the earth's crust.
It is in these three layers - bark, hydrosphere, atmosphere - where the right conditions for life to develop and maintain are given. Both water and air coverage are unique throughout the Solar System.
Water: the Hydrosphere
We call the whole hydrosphere all the water on the surface of the Earth: oceans, seas, rivers, lakes, swamps, glaciers, poles ... It was formed at an early time in Earth's evolution, from the steam produced by the volcanic eruptions, when they were more frequent than today. The vapor condensed into clouds that then caused torrential rains over millions of years. Noah's biblical story may pretend to explain this phenomenon, although, evidently, when it happened there were no humans.
Most of the water is found in the oceans, which cover almost three quarters of the earth's surface. In the northern hemisphere, the waters occupy about 154 million km. square, compared to 100 of the land that emerged. In the southern hemisphere, on the other hand, the seas occupy 206 million km. square, compared to only 48 million km. land squares.
On Earth there are about 1.4 billion km. cubic of water, of which, only 3.5% is fresh water and, of this, the majority is in the form of ice, at the poles. This huge amount of water helps to absorb the temperature differences that would occur in the different seasons of the year or between day and night.
Air: the atmosphere
Initially, the Earth had a very different atmosphere from the current one. Constant volcanic eruptions emitted huge amounts of water vapor that, as it precipitated, formed seas and oceans.
There arose the first algae that began to consume carbon dioxide and make oxygen. As the first one abounded and, however, there were no animals that consumed the second, the algae proliferated and, after millions of years, they had managed to transform the initial atmosphere into another one of composition similar to the current one.
The atmosphere is not uniform. Most of the air is concentrated in 15 km. closer to the earth's surface. From the ground, the atmosphere has different layers: troposphere, stratosphere, mesosphere, thermosphere, exosphere and magnetosphere. Due to the difference in densities, pressure and temperature between the different layers, or between different areas of the planet, the atmosphere presents constant changes that determine what we call "weather" or climate.
The atmosphere keeps the planet's temperature relatively stable and acts as a protective shield against various types of radiation that would be lethal to living beings. It also protects the earth's surface from the impact of meteorites, most of which, disintegrate when colliding with the upper layers of the atmosphere, at very high speeds.
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