The Atmosphere of Venus

The atmosphere of Venus is mostly carbon dioxide, 96.5% by volume. Most of the remaining 3.5% is nitrogen. Early evidence pointed to the sulfuric acid content in the atmosphere, but we now know that that is a rather minor constituent of the atmosphere.

The mass of the Venus atmosphere is about 90 times that of the Earth's atmosphere. 90% of the Earth's atmosphere is within 10 km of the surface, whereas you have to go to 50 km to capture 90% of the atmosphere of Venus. The clouds of Venus may extend from about 50 to 70 km and may be divided into three distinct layers. Below the clouds is a layer of haze down to about 30 km and below that it is clear.

Above the clouds there is a high-speed "jet stream" which blows from west to east at about 300-400 km/h. This wind is fastest at the equator and slows toward the poles, often giving a "V" type pattern in the visible cloud cover. At the surface there is almost no prevailing wind, with measured surface wind speeds typically less than 2 m/s.

High Temperature and Pressure on Venus

The temperature and pressure on the surface of Venus are so extreme that none of the armored Russian spacecraft of the Venera series lasted more than an hour on the surface. Within that short period, they and the multiprobe of the Magellan mission have provided us with all the direct data we have about the harsh Venusian surface. The pressure at the surface is about 90 Earth atmospheres! This is a pressure of about 900 Newtons per square centimeter or about 1300 pounds per square inch.

The surface temperature of about 750 K or about 480°C or nearly 900°F is hot enough to melt lead, hotter than any household oven temperature. This temperature destroyed the well-insulated and highly protected Russian electronics of the Venera surface craft within an hour.

Absence of Water on Venus

Since Venus is so similar to Earth in size and composition, one would expect a lot of water there, yet it is virtually absent. Because of the great similarities between the two planets, one would surmise that it must have had water in the past. Models for the loss of the water involve the runaway greenhouse effect on Venus. Being closer to the Sun and having the carbon dioxide in the atmosphere, the atmosphere continued to heat, perhaps with enhanced trapping of heat due to water vapor. With sufficiently high temperatures, the water vapor could rise high enough in the atmosphere for the water molecules to be broken up by ultraviolet radiation from the Sun. The freed hydrogen could then escape from the atmosphere, leaving the oxygen only in the form of carbon and sulfur oxides.

The upper atmosphere of Venus is more heavily bombarded by the solar wind since it has no magnetic field to redirect some of the solar wind particles. This would hasten the breakup of the water molecules if the above model is correct.

Sulfuric Acid Clouds on Venus

The benign images evoked by the name Venus are quickly dispelled by the notion of sulfuric acid clouds. Infrared observations starting in the 1970s indicated the presence of sulfur dioxide in the upper cloud layers and this was confirmed by later spacecraft observations. There may also be suspended sulfur particles in the upper atmosphere, contributing to the slightly yellow color of the visual images.

Presumably early volcanic activity contributed the sulfur to the atmosphere and the temperatures remained so high that it could not be trapped out into solid compounds on the surface as it did on the Earth. The melting point of sulfur is 386K, so the surface temperature of about 750K on Venus is well above the melting point. It is even above the boiling point of sulfur on the Earth (717K). Of course, in the 90 Earth-atmosphere pressure cooker of Venus, the sulfur would not be boiling, but it would be volatile enough to evaporate and form the sulfur dioxide compounds which would then remain airborne.

Besides the sulfuric acid clouds in the high atmosphere of Venus, there is evidence for sulfur dioxide in the haze which persists from 30-50km in the atmosphere. Despite the ominous-sounding presence of the sulfuric acid, it is a very minor constituent in the atmospheric composition of Venus.

Why So Much Carbon Dioxide on Venus and Not on Earth?

With a name like Venus, one expects the planet to be heavenly, but it is more like hell! With hotter-than-an-oven temperatures and 90 times the Earth's pressure, it's 96% carbon dioxide atmosphere gives us an example of the greenhouse effect gone wild! But a glance at the planet data table for Venus makes it clear that it is very much like the Earth. So why doesn't the Earth have a lot of carbon dioxide in the atmosphere?

One straightforward answer is that the carbon dioxide which may have been present in greater concentrations in the Earth's early atmosphere got dissolved into the oceans and trapped into solid compounds in the surface minerals, so that nearly all of it is gone from the present atmosphere. In the frightening scenario of a runaway greenhouse effect on the earth, which transformed all the presently trapped carbon into gaseous carbon dioxide, the Earth could become similar to the present Venus. So the vast differences in the current states of the two planets arise from rather small differences in initial temperatures and atmospheric concentrations. The heat-trapping by the Earth's atmosphere was not sufficient to lead to a progressively rising temperature. The profoundly different histories of the Earth and Venus should cause us to be attentive to the prospects of global warming.