Which statement about the thermosphere is true?

• A. The lowest layer of the atmosphere
• B. The last layer of the atmosphere that begins above the mesosphere and gradually fades into space
• C. A layer containing the ozone layer
• D. A layer with high atmospheric density

Answer: (B)

The outermost portion of the atmosphere is the thermosphere, which begins above the mesosphere and gradually fades into space. In this region the air is extremely thin, so the density is very low, even though temperatures can become very high with solar radiation. That high temperature does not feel hot to an object because there are so few molecules to transfer heat.

This region also hosts the ionosphere, which affects radio waves and is where auroras commonly occur, and it’s the area where many satellites orbit because there is less atmospheric drag. The ozone layer, by contrast, is located in the stratosphere, not the thermosphere. And the thermosphere does not have high atmospheric density; its air is far too sparse to be considered dense. The lowest layer is the troposphere, where weather happens, not the thermosphere.

So, the statement that describes the thermosphere as the outermost layer that begins above the mesosphere and gradually fades into space is the accurate one.