The correct option is (a).
Explanation
The Earth's atmosphere is divided into five distinct layers based on thermal properties and composition. The exosphere represents the transitional zone between the Earth's atmosphere and the vacuum of outer space.- Statement 1 is Correct: The exosphere is the uppermost layer of the atmosphere. It lies above the thermosphere and extends from approximately 600 km altitude up to 10,000 km, where it gradually merges with interplanetary space.
- Statement 2 is Incorrect: The air in the exosphere is extremely rarefied (thin), possessing very low density. It does not contain dense air; rather, the density is so low that particles travel hundreds of kilometers without colliding. Satellites orbit in this layer specifically because the friction (atmospheric drag) is negligible, not because the air supports them.
- Statement 3 is Incorrect: Heavy gases such as nitrogen and oxygen are concentrated in the lower layers of the atmosphere (Troposphere and Stratosphere) due to gravity. The exosphere contains light gases like helium and hydrogen, which float into space from this layer.
Key Takeaway: The exosphere is the outermost atmospheric layer characterized by extremely low air density and the presence of light gases like hydrogen and helium, distinct from the heavy gas concentration found in the lower atmosphere.