Venus has the densest atmosphere among the terrestrial planets. Its atmosphere is overwhelmingly carbon dioxide, with nitrogen making up most of the remainder, plus trace gases such as sulfur dioxide.
The atmosphere is extraordinarily compacted by Venus’s gravity and mass. Surface pressure is about 92–93 times Earth’s, comparable to the pressure nearly 900 metres beneath Earth’s oceans. The surface air is also roughly 50 times denser than Earth’s atmosphere at sea level.
This thick carbon-dioxide blanket drives an extreme greenhouse effect. Venus’s average surface temperature is about 462–467 °C, making it hotter than Mercury even though Venus is farther from the Sun. Sulfuric-acid clouds completely encircle the planet and hide its surface from ordinary visible-light observation.
A common mix-up is confusing atmospheric density with planetary density. Earth is the densest terrestrial planet overall, while Venus has the densest atmosphere. Mars has a mostly carbon-dioxide atmosphere too, but it is extremely thin; Mercury has only a tenuous exosphere, and Earth’s atmosphere is far less massive.