Lessons from Venus

by Rasmus Benestad and Ray Pierrehumbert

Venus Express will make unprecedented studies of the largely unkown phenomena taking place in the Venusian atmosphere. Credits: ESA - AOES Medialab A special report in The Observer on Sunday (April 9) titled ‘Venus – The Hot Spot’, provides a well-written account on a mission called the Venus Express. The Venus express is an European Space Agency (ESA) mission to probe the the atmosphere of Venus and address questions regarding the differences between the climates on Venus and Earth. According to the plans, the probe will enter the final orbit around Venus in May 2006, i.e. within about a month.

What relevance does a mission to Venus have for a blog like RealClimate? Primarily, Venus offers scientists the chance to see how the same basic physics used to study Earth’s climate operates under a very different set of circumstances. In one sense, Venus is rather similar to Earth: it has nearly the same mass as Earth, and while its orbit is somewhat closer to the Sun, that effect is more than made up for by the sunlight reflected from Venus’ thick cloud cover. Because of the cloud cover, the surface temperature of Venus would be a chilly -42C if were not for the greenhouse effect of its atmosphere. In reality, the surface of Venus, at 740K (467C) is even hotter than the surface of Mercury, which is a (relatively!) pleasant 440K. Per unit of surface area, the atmosphere of Venus has as much mass as about 100 Earth atmospheres, and it is almost pure CO2. This accounts for its very strong greenhouse effect. In contrast, the CO2 in the Earth’s atmosphere accounts for a mere .00056 of the full mass of one Earth atmosphere.

Despite the fact that Venus has vastly more CO2 in its atmosphere than Earth, the same basic principles govern the operation of the greenhouse effect for both planets: the fact that air cools by expansion as it rises means that the upper parts of the atmosphere are colder than the surface, while the opacity of greenhouse gases to infrared means that infrared radiation can only escape from the upper portions of the atmosphere. Since the rate of radiation goes down with temperature, the net effect allows the planet to lose energy at a rate much lower than it would if the radiation from the surface escaped directly to space. Although most of the warm surface temperature of Venus is accounted for by its CO2 greenhouse effect, there are suggestions that it is warmer than it should be on the basis of CO2 alone. There are various theories that have been proposed for the source of the additional greenhouse effect, and sorting this out will be one of the major objectives of Venus Express.

Another interesting difference between Venus and Earth is that Venus has a very slow rotation rate, taking 243 Earth days to complete one rotation. This is actually somewhat longer than its year. Despite the low rotation rate of the surface, the upper atmosphere is whizzing along at a rate of about one rotation every four Earth days. Pinning down the mechanisms responsible for this super-rotation will teach us much about atmospheric dynamics in general. In particular, it would be interesting to know if there are circumstances in which the Earth’s atmosphere could kick over into super-rotation.

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