Photospheric temperature of a Venus analogue (in terms of mass and radius), for a range of surface temperatures (x-axis) and surface pressure (y-axis). The atmospheric composition is assumed to be 100% CO₂. The top plots utilize a high CO₂ opacity, while the bottom plots are calculated using our nominal value that is 100 times lower (see discussion in Elkins-Tanton 2008). The left plots use the nominal linear dependence of the opacity on the pressure, while the right plots show the difference when using a pressure-independent opacity instead. Our atmosphere model becomes invalid below the horizontal black line, which indicates the transition to an optically thin atmosphere. The effective temperature of Venus (220 K, when taking into accounts its relatively high current opacity) is indicated with a thick line and the temperature and surface CO₂ pressure with an asterisk. We choose therefore the low opacity value and linear pressure dependence to best reproduce the atmosphere of Venus in our calculations.