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THE EARTH IS surrounded by a blanket of air, called the atmosphere. The atmosphere is a thin layer of gases that envelope the Earth. The gases are held close to the earth by gravity and the thermal movement of air molecules. Life on Earth is supported by the atmosphere, solar energy, and the magnetic fields. Five layers have been identified in the atmosphere, using thermal characteristics, chemical composition, movement, and density. The atmosphere is divided into the troposphere, the stratosphere, the mésosphère, the thermosphere, and the exosphere. The thermosphere, from the Greek word (thermos) for heat, is the fourth atmospheric layer from Earth, separated from the mésosphère by the mesopause. It begins about 50 mi. (80 km.) above the Earth and is the layer ...

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