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answers:
jake say: Thick carbon that keeps getting thicker. The temperature rises ever day and will continue to rise
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Gray Bold say: Venus has an extremely dense atmosphere composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases, most notably sulfur dioxide. The mass of its atmosphere is 93 times that of Earth's, whereas the pressure at its surface is about 92 times that at Earth's—a pressure equivalent to that at a depth of nearly 1 kilometre (0.62 mi) under Earth's oceans. The density at the surface is 65 kg/m3, 6.5% that of water or 50 times as dense as Earth's atmosphere at 293 K (20 °C; 68 °F) at sea level. The CO 2-rich atmosphere generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least 735 K (462 °C; 864 °F). This makes Venus's surface hotter than Mercury's, which has a minimum surface temperature of 53 K (−220 °C; −364 °F) and maximum surface temperature of 693 K (420 °C; 788 °F), even though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25% of Mercury's solar irradiance. This temperature is higher than that used for sterilization. The surface of Venus is often said to resemble traditional accounts of Hell.
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busterwasmycat say: well, ultimately because of PV=nRT (P is proportional to T, and P is very high, 90 times that of earth). the pressure is very high because Venus has a thick and dense atmosphere. High pressure imposes high temperature. Some also argue that it is the high CO2 content and its greenhouse effect that is responsible for the high temperature, but that is not really the main cause. It contributes, but the greenhouse effect was maxed out way before the content got into tens of weight percent CO2. Adding more CO2 does not cause more absorption of energy; all the energy in the absorption bands gets absorbed way before high concentrations are achieved. Only a portion of the radiant heat in any system is within the wavelength ranges "captured" by CO2, and that energy outside those wavelength ranges continues to radiate without regard to CO2 content. It actually increases as temperature increases, naturally.