The layers of gas that surround the Earth are collectively referred to as the atmosphere. The atmosphere is subdivided into five layers: the troposphere, stratosphere, mesosphere, thermosphere and exosphere. The outermost layer is the exosphere, and the innermost layer is the troposphere.
The atmosphere is composed of approximately 79 percent nitrogen and 21 percent oxygen, with small amounts of carbon dioxide and other trace gases present. The innermost layer, known as the troposphere, is where the atmosphere is the densest. This layer extends between four and 12 miles outward from the surface of the Earth, and it is where weather occurs.
Outside the troposphere is the stratosphere, which extends out to about 35 miles above the Earth's surface. The air is much lighter, and the temperature is below freezing. Because there is little turbulence in the stratosphere, planes often fly in this layer.
The mesosphere is less dense, extends 50 miles above the Earth's surface and reaches temperatures as low as -184 degrees Fahrenheit. Meteors often burn up in this layer. In the thermosphere, which extends to 400 miles above the Earth's surface, molecules are spread out and temperatures reach 3,600 degrees Fahrenheit. The outermost layer, the exosphere, extends to 6,200 miles above the Earth's surface. Gas molecules are few and far between there, and they easily escape into space.