Correct Option
The correct option is (c).
Explanation
The atmosphere is a vast blanket of gases surrounding the Earth, held in place by the planet's gravitational force. It is integral to life on Earth, providing necessary gases and regulating temperature. Its structure, density, and composition vary significantly with altitude.
Statement-wise Analysis
- Statement 1 is Incorrect. While the bulk of the atmosphere's mass is close to the surface, the atmosphere extends far beyond 100 kilometres. According to standard geographical texts, the atmosphere extends up to a height of about 1,600 kilometres. The 100 km mark (Kármán line) is often cited as the boundary between Earth's atmosphere and outer space for astronautical purposes, but the atmosphere physically extends much higher into the exosphere.
- Statement 2 is Correct. The atmosphere is held by gravity, which pulls gas molecules toward the Earth's surface. Consequently, the density of the atmosphere is maximum at sea level due to the weight of the air above compressing the lower layers. As altitude increases, the air becomes thinner, and density decreases rapidly.
- Statement 3 is Correct. The composition of clean, dry air is dominated by two gases. Nitrogen constitutes approximately 78 per cent of the atmosphere by volume, while Oxygen makes up about 21 per cent. The remaining 1 per cent consists of Argon, Carbon Dioxide, and other trace gases.
Key Takeaway
The atmosphere extends up to approximately 1,600 km, not just 100 km. Its density is highest at sea level and drops with altitude, while Nitrogen (78%) and Oxygen (21%) are its primary constituents.