As the highest layer of the atmosphere, the Earth's exosphere contains very thin air and features both lower and upper boundaries. The exosphere's boundaries vary by altitude depending on the level of the sun's activity and solar radiation emissions. There is some debate among scientists as to whether the exosphere should be considered a layer of the atmosphere or a part of space.
The exosphere is also known for not having many molecular collisions and for having low molecular densities. The Earth's exosphere contains the following gases: carbon dioxide, hydrogen, oxygen and helium. Below the Earth's exosphere is the thermosphere, followed by the mesosphere, stratosphere and troposphere.