Which gas primarily composes the atmosphere of Mars?

The atmosphere of Mars is primarily composed of carbon dioxide, making it the correct answer. Approximately 95% of the Martian atmosphere is comprised of this gas, which has significant implications for both the planet's climate and any potential for life.

Carbon dioxide is a greenhouse gas, but it is present in such a thin atmosphere on Mars that it does not create a substantial greenhouse effect compared to Earth. The substantial presence of carbon dioxide also plays a crucial role in the planet’s surface processes, influencing both its weather patterns and geological features.

In contrast, the other gases listed, such as oxygen, hydrogen, and nitrogen, are found in much smaller quantities in Mars' atmosphere. Oxygen, which is vital for life as we know it, makes up only about 0.13% of the atmosphere, while nitrogen contributes roughly 2.6%. Hydrogen exists as a trace gas on Mars and is even less prevalent than nitrogen and oxygen. Understanding these compositional differences is essential for discussions about Martian exploration and the challenges of sustaining human life there.