Which one of the following statements about the atmosphere is correct?

The Earth's atmosphere is characterized by a lack of a definite upper boundary. Instead, it gradually transitions into outer space through the exosphere, which is the uppermost layer extending beyond 400 km [2]. In this region, the air is extremely rarefied and molecules gradually 'leak' or dissipate into the vacuum of interplanetary space [2]. A fundamental physical property of the atmosphere is that its density is highest near the surface and decreases significantly with increasing altitude [2]. As one moves upward through the troposphere, stratosphere, mesosphere, and thermosphere, the atmosphere consistently thins out until the gases become imperceptible. While the KÝrmÝn Line is often used as a legal or functional boundary for space, physically, the atmosphere has no sharp outer limit and simply fades away [1].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Exosphere (700 to 1,000 km) > p. 279
2. [2] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 7: Composition and Structure of Atmosphere > STRUCTURE OF THE ATMOSPHERE > p. 65