Question

Which sub-layer thins out into space, where there is no air

Answer 1

The "exosphere" is the most distant and tenuous "layer" of our atmosphere.

Answer 2

The exosphere

Step-by-step explanation:

In Earth atmosphere, the closer to the ground is the troposphere, then the stratosphere, then the mesosphere, in which the highest clouds and radar can reach. Then in the thermosphere, where the auroras and the satellites are form, the air glow starts to disappear until we reach the space in the last layer called exosphere.