Which statement best describes the composition of Earth's early atmosphere?

• A. Nitrogen and oxygen
• B. Carbon dioxide and methane
• C. Hydrogen and helium with little oxygen
• D. Argon and neon

Answer: (C)

Early Earth drew its initial atmosphere from the same gases that filled the solar nebula, so the most abundant components were the light ones: hydrogen and helium. These gases were initially plentiful, but over time they largely escaped into space as the planet formed and the young Sun heated the atmosphere. Free oxygen hadn’t built up yet because there were no widespread photosynthetic organisms producing it. Other gases like carbon dioxide and methane did show up through outgassing, and nitrogen would come along later as well, but none topped the light H and He from the original envelope. That’s why describing the early atmosphere as hydrogen and helium with little oxygen best captures what was present at that stage.