Atmospheres in general are more transparent to high energy light than low energy light. The UV light from the sun penetrates the atmosphere and gets absorbed by some atom or molecule. That atom or molecule may emit a single photon of the same energy as was absorbed, or it may emit multiple photons, each of lower energy, but whose total energy is the same as the original UV photon. The final step is back to the beginning. The lower energy photons cannot penetrate the atmosphere to radiate back out into space, so their energy becomes a part of the thermal energy of the atmosphere. This is the greenhouse effect.
To summarize: UV light from the sun makes it through the clouds to the planet, but the IR light is reflected off. Then the UV light that made it through gets stepped down to IR light via absorption and emission. Now it's trapped inside the cloud layer, and will reflect back toward the surface. Even though 90% of the suns energy is reflected away into space (for Venus), the 10% that gets in is largely stuck there.
Note: if this was the whole story, Venus would be increasing in temperature without bound, which is clearly not the case. I suspect blackbody radiation of the upper atmosphere is what's missing. As the atmosphere heats, it emits more and more radiation. I.e. hot things glow brighter and brighter as they get hotter. My guess is that the atmospheric heating increases to the point where the blackbody radiation counters it, then it will hold that temperature.
If that process isn't enough to counter the incoming heat, then we could see evaporative cooling, like on Mercury.
Mercury has no atmosphere because it's so close to the sun. Any atmosphere it would accumulate is heated to the point where the average speed of the atmospheric molecules is greater than the tiny planet's escape velocity. The atmosphere effectively boils off.
Interestingly, Mercury is the planet with the greatest difference between it's max and min temperatures. Since it's orbital and axial rotations are fairly close (in a 3:2 relation), the day length is on par with the year length. Meaning that the planet has a really hot side and a really cold side. There are craters near the poles which are in permanent shadow, and extremely cold despite the proximity to the sun.
Is Venus burning up?
The Venusian atmosphere is full of CO2 and water, both greenhouse gasses, neither flammable. You get CO2 and water if you burn methane (natural gas). There's also a lot of sulfuric acid and other nasty stuff in the atmosphere. Even if the atmosphere were methane and oxygen, it's not clear that it burning would make it go away. It would depend on the amount of atmosphere, the mixing of the methane and oxygen, and the mass of the planet.
Or am I completely misunderstanding the part about Venus burning up?
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Originally Posted by MadMojoMonkey
