The Atmosphere of
Venus

The atmosphere of Venus is composed of about 96% carbon dioxide, with most of the remainder being nitrogen. The atmosphere appears to be relatively clear until the cloud deck starts about 50 km above the surface. The clouds are composed of sulphuric acid and various other corrosive compounds, and the atmosphere contains little water.

Absence of Water Vapor

The clouds contain little water vapor, and there is little evidence for water in any form on Venus. It is speculated that the absence of water is because most water that may have initially been on Venus made its way to the upper atmosphere where it was broken down by sunlight and interactions with cosmic rays and the solar wind into oxygen and hydrogen, which was then lost to interplanetary space.

There are several reasons to believe this. First, there is evidence that there was a lot of water on Venus at one time. This comes form Pioneer spacecraft observations which showed 150 times more DEUTERIUM (heavy isotope of hydrogen) per hydrogen atom than on Earth. This deuterium naturally forms from the dissociation of water (H20) into its respective components hydrogen and oxygen.

Because deuterium is a heavier isotope of hydrogen (having 1 neutron in the nucleus in addition to just the proton) it rises more slowly in the atmopshere. Thus the lighter isotope of hydrogen largely escaped but the heavier isotope remained. This provides evidence that there was more water on Venus. Enough to make a surface covering ocean about 25 meters deep.

The reason that the UV radiation could penetrate the atmosphere is because the oxygen was not able to form ozone but instead mainly formed oxides in the soil.

Now Venus is extremely dry and it is estimated that one could only form a surface covering ocean 0.3 m deep as compared with the Earth where one can form a surface covering ocean of 3000m deep.

Lower Atmosphere

The pressure of the atmosphere is about 90 times that of the Earth at the surface, and the surface temperatures on Venus are around 500 degrees Celsius, exceeding that of Mercury and hot enough to melt soft metals. Calculations indicate that for the temperatures to be so high there must be a mechanism in the Venusian atmosphere that traps solar radiation very effectively. As we will see in the next section, these rather remarkable properties of the atmosphere are thought to be a consequence of a runaway greenhouse effect. Note also that the density is only 10 times less dense the water. In the same way that you can "fly" in water (by swimming!) you could swim/fly in Venus atmosphere even though the gravity is close to that of Earth. You'd have to wear a space suit though.

Note that although the hot surface temperature of Venus is understood (see next section) the fact that there are high-velocity winds in the upper atmosphere but a relatively stagnant lower atmosphere with only very weak winds blowing at the surface is not well understood.