Venus is often cited by CO2 greenhouse advocates as a perfect example of “runaway greenhouse effect”, given its atmosphere is around 96% CO2, and the average surface temperature is a blistering 460°C. They claim it’s a perfect example of CO2 “greenhouse effect” (short waves in, long waves out and partly “reflected” back). Again, this seems to make sense, until you look closer. First, the atmosphere is thick and massive, nearly 100 times that of the earth. Secondly, it has a thick, highly reflective cloud layer (mostly sulfuric acid droplets) that reflects about 80% of sunlight. But there’s a little problem, the temperature at the surface and in the lower atmosphere remains fairly even night AND day (and a day on Venus lasts 243 earth days). This cannot be explained by the commonly accepted CO2 radiative “forcing” approach, otherwise there would be a significant drop in temperature at the surface during that long Venusian night as the planet radiates long wave radiation. This indicates the sun’s longer wavelengths don’t reach the planet itself; it seems the planet’s massive and dense atmosphere is such an effective damper that very little solar radiation above 0.05μm reaches the planet’s surface - in fact, the damper zone is probably from the upper atmosphere down to about 80km from the surface - below that altitude, the atmosphere maintains a fairly even temperature night and day; above 80km, the temperature varies (graph - Royal Belgian Institute). This seems to indicate that the damper zone on Venus is from the edge of the atmosphere to about 80km from the surface - and interestingly, the temperature and pressure at that height is about the same as here on earth at the surface. In other words, the Venusian atmosphere can absorb most of the sun’s heat energy by the time it reaches ~80km above the planet’s surface*. This could explain the unusual, high-speed air current below that level. But then, why is it so hot at the surface.? First, there are clear signs of ongoing volcanic activity, an indication Venus is geologically active, with a hot molten core and lava chambers close to the surface, releasing an incredible amount of energy. And then there is that very dense atmosphere and thick cloud layer that effectively trap that heat in; and then, that low-level high-speed air current preventing normal vertical convection cells from forming and carrying that heat up higher in the atmosphere. (I contend that the fact that the atmosphere is composed mostly of CO2 helps keep much of the solar radiation from reaching the planet surface and prevents it from getting even hotter than it is now). And then, there’s no water on the surface, so no phase change heat transfer to the atmosphere, only conduction and radiation, both not nearly as effective as phase change at transmitting heat to the atmosphere. In the end, internal heat is trapped in by that thick heavy blanket - and it wouldn’t make much difference what material the blanket was made of. At the poles, the atmosphere is less dense and .

*Note that a number of gas molecules (including CO2) can “intercept” incoming photons of any frequency; this can be quite effective when those gases are compressed to over 1,000 psi and the molecules are tightly packed together (this process is known as photolysis or photodecomposition). Also, “excited” CO2 molecules (remember, a couple of CO2’s absorption bands lie within the solar infrared range) can thermalize surrounding molecules when they collide (thermalization). Water vapor and aerosols are also present in the Venusian atmosphere; both are quite effective at capturing the sun’s heat radiation as well.

So it’s likely that the high surface temperature of Venus is mostly due to heat originating from within (radioactivity and latent heat from the planet’s formation), and the slow release of that heat at the planet’s surface, NOT from an “extreme greenhouse effect”. The absence of water on the planet’s surface also means there’s no heat transfer through phase change - only through much less-efficient conduction and radiation.