What gas makes up about 95 percent of Mars’s atmosphere?

Carbon dioxide makes up about 95 percent of Mars’s atmosphere.

Mars has a very thin atmosphere composed primarily of carbon dioxide, with smaller amounts of nitrogen and argon. The surface pressure averages only about 0.6 percent of Earth’s sea-level pressure, so the atmosphere provides little insulation and cannot support human breathing.

The carbon dioxide cycle strongly influences Martian weather. Carbon dioxide freezes into polar deposits during winter and sublimates back into the atmosphere during spring and summer. Dust storms can spread across large regions, and temperature differences can create powerful local winds despite the atmosphere’s low density.

Mars was probably warmer and wetter billions of years ago, when it had a thicker atmosphere and stable liquid water at the surface. The loss of much of its atmospheric protection allowed solar wind and radiation to affect the planet more strongly. Carbon dioxide is also important in discussions of future exploration, because spacecraft and habitats could potentially use atmospheric carbon dioxide to produce oxygen and fuel ingredients.