The least dense atmospheric layer is the exosphere. This layer is located farthest from the Earth’s surface, extending from about 600 kilometers (about 370 miles) to 10,000 kilometers (about 6,200 miles) above sea level.
In the exosphere, the atmosphere is incredibly thin, consisting mainly of light gases such as hydrogen and helium. Because the molecules in this layer are so sparse and spaced out, the density is extremely low compared to the layers below it, such as the thermosphere and mesosphere.
This low density means that particles in the exosphere can travel long distances without colliding with one another. In fact, it’s often considered where Earth’s atmosphere transitions into outer space.