What is the outermost layer of the thermosphere called?

The outermost layer of the thermosphere is known as the exosphere. This layer is characterized by a very thin atmosphere, where individual gas molecules can travel hundreds of kilometers without colliding with one another. The exosphere extends from the upper limit of the thermosphere to about 10,000 kilometers above the Earth's surface, gradually transitioning into the vacuum of space.

The exosphere is where satellites orbit the Earth, as the atmosphere is so sparse at this altitude that satellites can maintain their paths with little atmospheric drag. This layer primarily consists of hydrogen and helium, further distinguishing it from the other layers of the atmosphere, which have higher concentrations of heavier gases.

In contrast, the ionosphere, which is sometimes confused with the exosphere, is actually a region within the thermosphere that is ionized by solar radiation and is crucial for radio communication. Other layers like the stratosphere and mesosphere lie below the thermosphere and do not encompass the characteristics found in the exosphere.