Venus’s atmosphere is very, very dense. It is composed of about 96% carbon dioxide, 3.5% nitrogen, and trace amounts of other gases, including sulfur dioxide. Although Earth’s atmosphere is composed of over 75% nitrogen, Venus’s atmosphere is so dense that the 3.5% of its atmosphere that is composed of nitrogen has around 4 times the mass of the nitrogen found in Earth’s atmosphere. The density of the atmosphere on Venus also means that there is about 90 times the atmospheric pressure on Venus’s surface than on Earth’s, which is pressure similar to what would be experienced if you dove about 1000 meters under the surface of one of Earth’s oceans.
The composition of Venus’s atmosphere is also largely responsible for its surface/planetary heat. In the past, the increasing brightness of the early sun, as well as the composition of Venus’s atmosphere (carbon dioxide is a greenhouse gas, which warms the planet), contributed to a runaway greenhouse effect. This effect is essentially a positive feedback loop that caused the planet’s ocean(s) to evaporate and the surface temperature to rise higher and higher. Because of this, Venus’s surface is even hotter than Mercury’s, despite the fact that Mercury is closer to the Sun. The carbon dioxide, sulfur dioxide, and trace amounts of water vapor in Venus’s atmosphere also react with each other to form clouds made up of sulfuric acid, a corrosive compound that can result in severe burns and skin tissue damage; however, acid rain never reaches the surface of Venus since the intense heat evaporates it all before it can reach the surface.