This is a picture which shows the Earth, its atmosphere (the clouds are likely in the troposphere and stratosphere), the limb of the Earth (the dark blue curve/edge which is the mesosphere and thermosphere), and the dark blue to black region of space (where our exosphere extends out to...).
Click on image for full size
Image courtesy of NASA
The Exosphere
At very high altitudes, the atmosphere becomes very thin. The region where atoms and molecules escape into space is referred to as the exosphere. This is the true upper limit of the Earth's atmosphere. The exosphere extends from
the thermosphere out to space.
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