Venus has an atmosphere containing about 96.5% carbon dioxide by volume.
Venus is a rocky planet roughly similar to Earth in size, but its atmosphere is vastly denser and hotter. Most of the remaining atmosphere is nitrogen, at about 3.5%, with sulfur dioxide and other gases present in trace quantities. The heavy carbon-dioxide atmosphere produces an intense greenhouse effect that keeps the surface extremely hot.
The average surface temperature is about 465°C (869°F), hotter than Mercury’s average surface despite Venus receiving less sunlight. Its surface pressure is roughly 92 times Earth’s sea-level pressure, comparable to the pressure found deep beneath Earth’s oceans. At the surface, the pressure and temperature are high enough for carbon dioxide to behave as a supercritical fluid.
The key mix-up is between atmospheric composition and planetary identity. Earth’s atmosphere is mostly nitrogen and oxygen, Mars has a carbon-dioxide-rich but extremely thin atmosphere, and Jupiter is dominated by hydrogen and helium. Venus combines a very high CO2 percentage with a massive atmosphere.