Venus and the Runaway Greenhouse Effect


The high surface temperature of Venus was confirmed in the 1950s by radio astronomers and later by the Mariner 2 spacecrafts and Venera probes of the 1960s and 1970s. It would ordinarily be expected that the surface temperature of Venus should be within the range of that calculated from solar heating; which is generally below 200K. Yet the Soviet Venera craft which first landed on the surface of Venus found a planetary environment with a surface pressure of 90 bars and a temperature well over 700K. Scientists were for some time puzzled by this observation, but they soon found the answer in the greenhouse effect.

Venus has almost a million times more carbon dioxide than the atmosphere of our planet earth. Green house effect works on Venus exactly as it works on the planet Earth. Sunlight diffusing into the atmosphere of Venus heats its surface, but carbon dioxide acts as a blanket keeping infrared or heat radiation from diffusing out back into space. Consequently the surface of the planet heats up till a balance is established between the heat which manages to escape and the incoming solar radiation.

An intriguing question which arises from the what we know about the Venetian atmosphere is one that is of special interest to us on a our planet at a time in which carbon dioxide emissions consequent to human activities places us at a risk of significant increases in surface temperature of our planet .. Could it be that Venus had started with a more earthlike condition but somehow evolved, by mechanisms we fear might have been initiated in our planet, to its present state? This question is of special significance because in many respects Venus is a remarkably earthlike planet with a composition and size about that of the earth and with a geology significantly like ours (except for the significant exception of plate tectonics replaced on Venus by stationary or "blob "tectonics). A close study of the Venus atmosphere may therefore enlighten us on the reality or otherwise of our fear that human activity may transform the Earth into the hellish environment like Venus'.

Scientists have constructed a hypothetical scenario in which in Venus started off as an Earthlike planet but evolved by runaway greenhouse effect to the inhospitable world that it is today:

In the beginning Venus was an evnvironment with warm ocean in which much of the atmospheric carbon dioxide was dissolved and the balance chemically combined with surface rocks. A rise in energy output of the sun as it occurs, for instance, in solar flares might have led to a slight additional heating of the surface of the planet. This leads to a release of carbond dioxide from the oceans and from the rocks with which it is combined. The increase in carbon dioxide leads in turn to a further increase in temperature by the greenhouse effect, leading to still more carbon dioxide being released along with water into the atmosphere.

This process is repeated and temperature continues increasing in a vicious cycle ie more carbon dioxide in the atmosphere leads to further increase in temperature and consequently the release of more carbon dioxide, and so on …

This situation is what is termed the runaway greenhouse effect. The atmosphere finally re-establishes a new equilibrium at a much higher temperature. If the new equilibrium temperature is high enough the oceans may evaporate leading to an atmosphere of hot water vapor which in turn contributes further to greenhouse effect. Water vapor is, however, unstable in the presence of ultraviolet radiation which breaks it down into hydrogen and oxygen. Hydrogen being a light element escapes into space leaving only oxygen.

Scientists believe that Venus once had water but it was lost to the planet in a runaway greenhouse process as described ..

The relevant question is could this scenario be actualized on the planet Earth?

Source by John Thomas Didymus


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