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WHY IS VENUS SO HOT?

     Venus' atmosphere represents a runaway greenhouse effect. 96% of the composition of the atmosphere is carbon dioxide, 3.5% is nitrogen. The remaining is sulfuric acid, hydrochloric acid, and hydrofluoric acid. The thick clouds that cover the surface of Venus are probably composed of sulfuric acid and sulfur crystals.

     Spacecraft that have reached the surface have found temperatures of 470C (878F). These harsh conditions are mostly due to Venus being 30% closer to the Sun. Venus receives twice as much solar heating however. This apparently never allowed water to condense and therefore to form oceans. Without oceans there was no effective way to absorb carbon dioxide.

     What determines how hot a planet will be? A planet receives sunlight and some of it is absorbed by the surface which is then heated. This thermal energy is reflected back toward space as infrared radiation. If the amount of sunlight received is greater than the amount of infrared emitted, the planet heats up. If it is less, the planet cools.

     The planet's atmosphere absorbs some of the emitted infrared radiation. The composition of the atmosphere determines how much is absorbed. The more CO2 and H2O in the atmosphere, the more infrared it will absorb. This warms the atmosphere and the process will continue until the amount of infrared leaving the atmosphere equals the amount of incoming sunlight. On Venus, the temperature in the atmosphere must reach 750 K before equilibrium is achieved. .

Worse yet, the temperature has gotten so high that sulfur, fluorine and chlorine have been released out of the rocks. These gases have combined with existing elements to form hydrochloric, hydrofluoric, and sulfuric acid vapors in the atmosphere. When it does "rain" on Venus, it is a truly acid rain. None of this rain however, ever reaches the surface. The temperature is so hot that the acid droplets evaporate before they ever reach the ground.

Copyright © 1997 Kathy A. Miles and Charles F. Peters II