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Planet Earth, along with Mercury, Venus and Mars, is one of the telluric planets that are located close to the Sun and of a small size and heavy density. Venus and Mars, like Earth, possess an atmosphere. At present, the Earth’s atmosphere consists largely of nitrogen (78 %) and oxygen (21 %).
 The composition of the Earth’s atmosphere (click to enlarge)
© rue des écoles
Among the « other gases », are water vapour (between 1 and 4 %), neon (0,0018 %), helium (0,00052 %) and methane (0,00015 %).
The atmosphere is divided into superimposed layers. The troposphere and the stratosphere, in particular, are involved in the natural greenhouse effect.
The troposphere, situated at an altitude between 0 and 10 km, contains about 80 % of the total mass of the atmosphere and virtually all the water vapour. This layer is full of movement and it is here that meteorological phenomena such as rain and wind are observed. In the troposphere, the temperature decreases regularly by about 6 °C for every 1 000 metres of altitude.
The stratosphere is at an altitude of 10 to 50 km. It contains the ozone layer, also known as the ozonosphere. The ozone layer absorbs part of the solar radiation, resulting in an increase in temperature at this altitude. It also protects the biosphere from certain harmful radiation (ultraviolet radiation and cosmic rays).
The mesosphere is at an altitude of 50 to 80 km. It contains CO2 which can be involved in the greenhouse effect.
The thermosphere, the layer situated above an altitude of 80 km, can reach a temperature of 500 °C close to the edge of space. The pressure at this altitude is virtually zero and molecules of air are extremely rare. This layer also has an influence on the climate.
Earth is the only planet possessing a hydrosphere, a layer of water present in all its three states (solid, liquid and gaseous) (see picture 2).
Due to a combination of factors: its distance from the Sun (150 million kilometres), the conditions of pressure and temperature ruling on its surface and the presence of an atmosphere and a hydrosphere, the Earth is the only planet to possess a biosphere.
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