The reason Venus is hot has absolutely nothing to do with the 'greenhouse effect'. Gases must circulate in a negative temperature gradient because packets of gas are unstable under bouyancy forces. This means that as they circulate, they undergo a 450:1 compression. The process is adiabatic, giving a temperature rise from the tropopause to the surface of 204K to 740K. The 'greenhouse effect' is canals on Mars and Piltdown Man all over again. It is folklore, in the same category as the works of Hans Christiaan Andersen.