What gas makes up most of the atmosphere of Mars?

The gas that makes up most of Mars's atmosphere is carbon dioxide.

Mars has a very thin atmosphere composed primarily of carbon dioxide, which accounts for roughly 95% of the gas surrounding the planet. Nitrogen and argon make up most of the remainder, while oxygen, water vapor, carbon monoxide, and other gases appear only in small quantities.

Although carbon dioxide is a greenhouse gas, Mars's atmosphere is so thin that its warming effect is weak compared with Earth's. Surface atmospheric pressure on Mars is less than 1% of Earth's average sea-level pressure. This thin air cannot support human breathing and provides little protection from radiation or extreme temperature changes.

Mars's polar ice caps contain substantial frozen carbon dioxide, often called dry ice, as well as water ice. During winter, some atmospheric carbon dioxide freezes at the poles; during warmer seasons, it sublimates directly back into the atmosphere. The planet's reddish appearance is caused mainly by iron minerals in its soil, not by the carbon dioxide in its air. Venus also has a carbon-dioxide-dominated atmosphere, but it is vastly thicker and far hotter.