The Exosphere
where everything ends ...
The exosphere (after Greek: έξο, éxo: outer, outside) is the uppermost layer of the atmosphere and marks the transition into interplanetary space.
Its lower boundary at the edge of the thermosphere is about 500 km to 1000 km above the Earth's surface, and its upper boundary is at about 10,000 km altitude. This boundary is not well defined since air density decreases continuously. The main gases within the exosphere are the lightest gases, mainly hydrogen, helium, and atomic oxygen near the exobase. All compounds are ionised. The exosphere is the only layer wherefrom atmospheric gases can escape into space.
The high temperature of the exosphere is due to the high velocities of the gas particles (high velocities correspond to high temperatures). A thermometer, however, would show extremely low temperatures, since the density of the gas is much too low to cause a significant heat transport.
