The correct option is 2 only.
Explanation
The Earth's atmosphere is divided into five distinct layers based on thermal and compositional properties. The Exosphere constitutes the outermost layer, situated above the Thermosphere.
- Statement 1 is Incorrect: The Exosphere is the uppermost layer of the atmosphere. The lowest layer, which is in contact with the Earth's surface and contains the majority of the atmospheric mass, is the Troposphere.
- Statement 2 is Correct: The contents of the Exosphere are extremely rarefied. In this layer, the density of air is negligible, and the atmosphere is composed primarily of very light gases such as hydrogen and helium, which float into space.
- Statement 3 is Incorrect: The Exosphere does not possess a distinct or sharp upper boundary. Instead, it gradually merges with the vacuum of outer space. Due to the extremely low density of particles, the transition from the Earth's atmosphere to interplanetary space is indefinite.
Key Takeaway: The Exosphere is the highest layer of the atmosphere, characterized by extremely low air density and a gradual, undefined transition into outer space.