RAS question
Earth's atmosphere is composed primarily of:
Correct answer: (D) Nitrogen (78%) and Oxygen (21%).
Earth's atmosphere consists mainly of about 78% nitrogen and about 21% oxygen.
Explanation
According to NASA Science's Earth facts page, the atmosphere near Earth's surface is about 78% nitrogen, 21% oxygen, and 1% other gases, including argon, carbon dioxide, and neon. Nitrogen is therefore the dominant gas, and oxygen is the second-most abundant. More precise standard values are about 78.09% nitrogen, 20.95% oxygen, 0.93% argon, and 0.04% carbon dioxide; neon, helium, methane, and krypton occur in trace amounts. Hydrogen and carbon dioxide do not account for 78% of the atmosphere, and reversing the nitrogen and oxygen percentages also gives an incorrect composition.
Why the other options are wrong
- (A) Hydrogen is not the main gas in Earth's atmosphere; nitrogen accounts for roughly 78%.
- (B) Carbon dioxide accounts for about 0.04% of the atmosphere and is part of the small fraction of other gases, not 78%.
- (C) This pairing reverses the two major gases: oxygen is about 21%, while nitrogen is about 78%.
Concept
Atmospheric composition is a core physical geography fact used in climate, weather, and environmental questions. It recurs in RAS because gas composition links directly to greenhouse gases, atmospheric layers, and human-environment interactions.