Surface Temperature Regulation

If a planet has no water vapor in its atmosphere, then its surface temperature is determined primarily by the abundance of carbon dioxide in its atmosphere. Planet Venus has an atmosphere consisting of 98% carbon dioxide and surface pressure 90 times that of Earth. Solar energy, which is mostly visible radiation, passes through the window in the visible range (carbon dioxide has no absorption between 0.4 and 0.7 microns) and strikes the surface where, according to the table of albedos, 24% is converted to heat. The large amount of carbon dioxide in the Venus atmosphere produces a very strong greenhouse effect that re-radiates infrared energy back to the surface giving a surface temperature of 477^oC. Planet Mars, on the other hand, has at atmosphere of 95% carbon dioxide but a surface pressure less than 1/100 of that of Earth. This rather low total mass of carbon dioxide can only create a very weak greenhouse effect on Mars, and its surface temperature is -53^oC. The temperature of a planet's surface is more related to the amount of carbon dioxide in its atmosphere than to its position relative to the sun.

The important concept from this discussion is that greenhouse gases in general, and carbon dioxide in particular, regulate the temperature at the planet surface. The fact that earth has far less carbon dioxide than Venus and considerably more than Mars gives earth a unique range of surface temperature that is favorable for plant, animal, and human life as we know it.

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