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L'Atmosphère

What is a planet's atmosphere?
An atmosphere (from Ancient Greek ἀτμός (atmós) 'vapour, steam', and σφαῖρα (sphaîra) 'sphere') is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low.
How much does the atmosphere weigh?
The atmosphere has a mass of about 5.15 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.
What are the different layers of Earth's atmosphere?
The atmosphere of Earth is composed of layers with different properties, such as specific gaseous composition, temperature, and pressure. The troposphere is the lowest layer of the atmosphere. This extends from the planetary surface to the bottom of the stratosphere.
How does the atmosphere affect the surface of a rocky body?
Atmospheres have dramatic effects on the surfaces of rocky bodies. Objects that have no atmosphere, or that have only an exosphere, have terrain that is covered in craters. Without an atmosphere, the planet has no protection from meteoroids, and all of them collide with the surface as meteorites and create craters.

What is a planet's atmosphere?