Which planet has an atmosphere that is dense and composed largely of carbon dioxide?

• A. Earth
• B. Mars
• C. Neptune
• D. Venus

Answer: (D)

A dense atmosphere dominated by carbon dioxide means a thick air column near the surface that traps heat very effectively. Venus fits this description. Its atmosphere is mostly carbon dioxide, making it extremely dense—surface pressures around 90 times Earth's and surface temperatures about 450–470°C. The abundant CO2 creates a strong greenhouse effect, so heat from the Sun is trapped and can’t escape easily, leading to the scorching surface.

Earth, in contrast, has an atmosphere dominated by nitrogen and oxygen and is relatively thin compared to Venus. Mars has CO2 present but the atmosphere is very thin, so even though CO2 is abundant, it doesn’t create a dense air column. Neptune’s atmosphere is primarily hydrogen and helium with methane, not carbon dioxide. So the planet with a dense, CO2-rich atmosphere is Venus.