that Venus has a very dense atmosphere made up of carbon dioxide, nitrogen, and sulfuric acid, while Mercury has a very thin atmosphere with various gases, but very little carbon dioxide. Usually, the surface of a planet is warmed during the day and cools off at night by releasing infrared radiation (heat) back into space. But the carbon dioxide in Venus' clouds absorbs energy from infrared radiation very well and "traps" the heat on the planet, making it very warm. This has sometimes been called a "runaway greenhouse effect." We don't see this happen on Mercury because its atmosphere is not thick and does not have much carbon dioxide in it