Here's why:

* Troposphere: The lowest layer is the densest, containing about 75% of the atmosphere's mass.

* Stratosphere: The next layer up, the stratosphere, contains about 19% of the atmosphere's mass. This is significantly less than the troposphere, but still higher than the other layers.

* Mesosphere: The mesosphere is much less dense than the stratosphere.

* Thermosphere: The thermosphere is very thin, with very few molecules present.

* Exosphere: The outermost layer is the exosphere, where the atmosphere gradually fades into space.