The Earth and Venus are near each other in the Solar System, and are similar in size, density, and composition. Based on our understanding of the origin of the Solar System, we would expect that their initial atmospheres would have been rather similar. Yet the present atmospheres of the two planets could hardly be much more different than they are. How did this come to be? The reason is thought to lie in what is termed the “Runaway Greenhouse Effect”.
Sunlight falling on the surface of a planet is primarily in the visible part of the spectrum. However, the reflection of light from the surface tends to produce light of longer wavelength called infrared (IR) radiation (also known as radiant heat; IR radiation is the heat that we sense being radiated from a hot surface like a hot piece of metal).
Now, because of their molecular structures, certain gases like carbon dioxide and water vapor (and many others) have the property that they are essentially transparent to visible light but absorb IR radiation very strongly. Such compounds are sometimes termed greenhouse gases because, if they are present in a planetary atmosphere, they absorb the scattered IR radiation and tend to raise the temperature of the atmosphere by trapping solar energy. (The analogy with a real greenhouse is imperfect because the mechanism by which a greenhouse stays warm is different, but it is sufficiently good that the name “(Planetary) Greenhouse Effect” is now the common one for this phenomenon.)
The greenhouse effect occurs for all planetary atmospheres containing greenhouse gases, and is responsible for their being warmer than would be the case otherwise. The greenhouse effect by itself could not account for the conditions that we find on Venus. However, under certain conditions we believe the greenhouse effect can “run away”. For example, consider the case of a planet like the Earth. The Earth has enormous amounts of two greenhouse gases: water vapor and carbon dioxide. However, for the Earth most of the water and carbon dioxide are not in the atmosphere. The water is mostly in the oceans, and the carbon dioxide is mostly bound chemically in rocks made from compounds that chemists call carbonates (for example, limestone).
Now suppose we increased the effectiveness of greenhouse heating of the Earth’s atmosphere, for example by increasing the amount of solar radiation falling on it, or by increasing the concentration of greenhouse gases in the atmosphere (for example, by burning fossil fuels, which produce water vapor and carbon dioxide as byproducts of burning). We would then expect the temperature to rise in the atmosphere (assuming no other effects intervened—a big “if” in the realistic case since the atmosphere is complicated). This would be a greenhouse effect.
It would become a runaway greenhouse effect if the rising temperature approached the boiling point of water, because then the oceans would begin to convert to water vapor, the water vapor would increase the effectiveness of heat trapping and accelerate the greenhouse effect, this would cause the temperature to rise further, thus causing the oceans to evaporate faster, etc., etc. (This type of runaway is also called a “positive feedback loop”.) When the oceans were gone the atmosphere would finally stabilize at a much higher temperature and at much higher density, because all the water would now be in the atmosphere.
We can envision even a further runaway stage in this scenario. Suppose the preceding runaway raised the temperature so high that chemical reactions begin to occur that drive the carbon dioxide from the rocks into the atmosphere (the process is called sublimation; a few hundred degrees Celsius would be sufficient). Then another runaway would occur as the carbon dioxide feeding into the atmosphere would accelerate the heating, which would in turn accelerate the transfer of carbon dioxide from the rocks to the atmosphere.
The atmosphere would finally stablilize at a still higher temperature and pressure after all the carbon dioxide had been driven from the rocks. In fact, we believe that if this sequence were to take place on the Earth, the resulting temperature and pressure of the atmosphere left behind would not be very different from that for present-day Venus: the atmospheric termperature would be hundreds of degrees Celsius and the pressure would be maybe 100 times greater than it is today.
Thus, we believe that in the case of Venus the initial solar heating kept oceans from forming, or kept them from staying around if they did form, and the subsequent lack of rainfall and failure of plant life to evolve kept the carbon dioxide in the atmosphere rather than binding it in the rocks as is the case for the Earth; thus, Venus has an environmental disaster for an atmosphere.
The sobering warning for us is obvious: we have to be extremely concerned about processes such as burning of fossil fuels in large volumes that might (we don’t know for sure because the scientific questions are complex) have the potential to trigger a runaway greenhouse effect and produce on the Earth atmospheric conditions such as those found on Venus.
