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Our atmosphere is divided into a number of layers. These include troposphere, stratosphere, mesosphere, thermosphere and exosphere. They exist between ground level to 4-12 miles, 4-12 miles to 31 miles, 31 miles to 53 miles, 53 miles to 375 miles and 375 miles to 6200 miles above earth's surface, respectively. The exosphere is the topmost layer and extends from the boundary of thermosphere to outer space. Exosphere contains molecules that are bound to Earth by gravity, but such attraction is so weak that they stop behaving like gas. Most of these molecules move around without hitting each other and some of them may actually escape to the space. Due to such conditions and lack of gaseous matter (or air), the air pressure in exosphere is practically zero. It is difficult to get a definite value of such pressure in milibars, since the boundaries of exosphere are not very clearly delineated. But for all practical purposes, we can assume the air pressure in exosphere to be zero (or very very close to it).
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