Chemistry
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Air and atmosphere
Atmosphere
1) What is an atmosphere ?
Definition of an atmosphere: this is a layer of gases surrounding a celestial body.
Examples of celestial bodies that have an atmosphere: Earth, Mars, Venus, Jupiter, the Sun, Titans.
Examples of celestial bodies with no atmosphere: the Moon, Mercury, Pluto.
Comment: The presence of an atmosphere around a celestial body doesn't systematically means that it can sustain life.
2) Characteristics and layers of Earth's atmosphere.
The Earth's atmosphere is the layer of air surrounding the Earth.
Its thickness is estimated at about 500 km.
Temperature variations in Earth's atmosphere are not regular. In some areas it decreases and it increases in others. This leads to distinguish several layers:
- The troposphere from 0 to (on average) 12 km in altitude. The temperature decreases with altitude. The troposphere contains about 90% of the air.
- The stratosphere extends roughly from 12 to 50 km altitude. The temperature is steadily increasing.
- The mesosphere extends roughly from 50 to 80 km altitude. The temperature is steadily increasing.
- Thermosphere extends roughly from 80 to 500 km
Comment: sometimes an additional layer called the exosphere is included in atmosphere. Exosphere is a transition area to interplanetary space that it is still protected from particles emitted from the Sun, by the Earth's magnetic field.
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