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The Greenhouse Effect on Venus

The nitrogen and oxygen gases that make up the bulk of Earth's atmosphere neither absorb nor emit thermal radiation, energy that is released from hot surfaces as electromagnetic waves. It is the water vapor, carbon dioxide, and some other minor gases present in the atmosphere that absorb some of the thermal radiation leaving the surface, acting as a partial blanket for this radiation and causing the difference between the actual average surface temperature on Earth (about 15°C) and the expected temperature if the atmosphere contained nitrogen and oxygen only (about -6°C). This blanketing is known as the natural greenhouse effect, and the gases are known as greenhouse gases.

Greenhouse effects similar to those on Earth also occur on our planetary neighbors, Mars and Venus. Mars is smaller than Earth and possesses, by Earth's standards, a very thin atmosphere. A barometer on the surface of Mars would record an atmospheric pressure less than 1 percent of that on Earth. Its atmosphere, which consists almost entirely of carbon dioxide, contributes a small but significant greenhouse effect.

The planet Venus, which can often be seen fairly close to the Sun in the morning or evening sky, has a very different atmosphere from Mars's. Venus is about the same size as Earth. A barometer for use on Venus would need to survive very hostile conditions and would need to be able to measure a pressure about one hundred times as great as that on Earth. Within Venus' atmosphere, which consists very largely of carbon dioxide, deep clouds consisting of droplets of almost pure sulfuric acid completely cover the planet and prevent most of the sunlight from reaching the surface. Some Russian space probes that have landed there have recorded what would be dusk-like conditions on Earth—only 1 or 2 percent of the sunlight present above the clouds penetrates to the surface. One might suppose, because of the small amount of solar energy available to keep the surface warm, that it would be rather cool; on the contrary, measurements from the same Russian space probes find a temperature there of about 525°C—a dull red heat in fact.

The reason for this very high temperature is the greenhouse effect. Because of the very thick absorbing atmosphere of carbon dioxide, little of the thermal radiation from the surface can get out. The atmosphere acts as such an effective radiation blanket that, although there is not much solar energy to warm the surface, the greenhouse effect amounts to nearly 500°C.

What occurs on Venus is an example of what has been called the "runaway" greenhouse effect. [A] It can be explained by imagining the early history of the Venus atmosphere, which was formed by the release of gases from the interior of the planet. [B] At first the atmosphere would contain a lot of water vapor, a powerful greenhouse gas. [C] The greenhouse effect of the water vapor would cause the temperature at the surface to rise. [D] The increased temperature would lead to more evaporation of water from the surface, causing more atmospheric water vapor, a larger greenhouse effect, and therefore a further increased surface temperature. The process would continue until either the atmosphere became saturated with water vapor or all the available water had evaporated.

A runaway sequence something like this seems to have occurred on Venus. Why, we may ask, has it not happened on Earth, a planet of about the same size as Venus and, so far as is known, of a similar initial chemical composition? The reason is that Venus is closer to the Sun than is Earth; the amount of solar energy per square meter falling on Venus is about twice that falling on Earth. The surface of Venus, when there was no atmosphere, would have started off at a temperature of just over 50°C. Throughout the sequence described above for Venus, water on the surface would have been continuously boiling. Because of the high temperature, the atmosphere would never have become saturated with water vapor. Earth, however, would have started at a colder temperature; at each stage of the sequence, it would have arrived at an equilibrium in which the oceans never became hot enough to evaporate.

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In general, this means that a chain of events was set in motion, with each event building on the previous one to continually raise temperatures.

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