Outer layer blending into space with extremely low density?

• A. Troposphere
• B. Stratosphere
• C. Mesosphere
• D. Exosphere

Answer: (D)

The outer atmosphere gradually thins and blends into space, with extremely low particle density. This outermost region is the exosphere. In the exosphere, collisions between particles are exceedingly rare, and individual atoms or molecules can travel long distances, with some escaping Earth's gravity into space. There isn’t a sharp edge—just a gradual thinning into interplanetary space—which is why it’s described as blending into space. For context, the lower layers have progressively higher densities and well-defined characteristics: the troposphere contains weather and clouds, the stratosphere houses the ozone layer and warms with altitude, and the mesosphere is where meteors burn up.