Question 53

The exosphere is the least dense atmospheric layer.

The exosphere is characterized by its extremely low density, where the air is so thin that individual particles can travel hundreds of kilometers without colliding with one another.

A) Exosphere

The exosphere is the outermost layer of the Earth's atmosphere, extending from about 600 kilometers above sea level to 10,000 kilometers. In this layer, the density of air is incredibly low, making it the least dense compared to all other atmospheric layers.

B) Mesosphere

The mesosphere lies below the thermosphere and above the stratosphere, extending from about 50 to 85 kilometers above the Earth's surface. Although it is less dense than the troposphere and stratosphere, it is still denser than the exosphere.

C) Troposphere

The troposphere is the lowest layer of the atmosphere, extending from the Earth's surface up to about 12 kilometers. It contains the majority of the atmosphere's mass and is significantly denser than the exosphere, making it an incorrect choice for the least dense layer.

D) Thermosphere

The thermosphere is located above the mesosphere and can extend from about 85 kilometers up to 600 kilometers. While it has low density due to its altitude, it is still denser than the exosphere, particularly because of the presence of ionized particles.

Conclusion

The exosphere is definitively the least dense atmospheric layer due to its extreme altitude and sparse particle distribution. In contrast, the other layers—mesosphere, troposphere, and thermosphere—contain more air molecules, resulting in higher density. Therefore, A is the only correct answer in this context.