MadSci Network: Astronomy

Re: Greenhouse-effect on Venus

Date: Mon Nov 30 14:09:06 1998
Posted By: Jason Goodman, Graduate Student, Massachusetts Institute of Technology
Area of science: Astronomy
ID: 910803086.As

Yes, our current understanding of Venus suggests that it experienced a "runaway greenhouse effect" at some point in its history. This led to the hostile conditions we see today.

Venus currently has a thick atmosphere of carbon dioxide 95 times the weight of Earth's atmosphere (95 "bars"), and its surface temperature is over 450 degrees C. There is essentially zero water anywhere on the planet or in its atmosphere. But in the early days of the solar system, we believe Venus probably had almost as much water as Earth did, and the amount of sunlight reaching it was only 30% greater than the sunlight reaching Earth today. What happened?

The amount of water which can evaporate into the air depends on the temperature. There can be more water vapor in hotter air. However, water vapor is a very powerful "greenhouse gas": it prevents heat from escaping to space. More water vapor will cause the surface temperature of the planet to rise -- this causes even more water to evaporate. So hotter temperatures cause more water vapor, which causes even hotter temperatures, and even more water vapor, and so on. Under certain conditions, this can cause a "runaway" chain reaction, which continues until all the water on the planet has been evaporated. This causes a "steam atmosphere" -- a lethal sauna. The slightly greater amount of sunlight received by Venus was apparently enough to cause this to happen.

But why don't we see any water in Venus's atmosphere today? When water vapor exists in the upper atmosphere of a planet, ultraviolet radiation from the sun can break apart the water molecules into hydrogen and oxygen. The hydrogen is light enough to fly away from the planet, leaving the oxygen behind -- the oxygen then reacts with rocks and disappears from the atmosphere. It seems that Venus lost all its water that way.

On planets which contain water (like Earth), water, carbon dioxide, and silicate rocks can interact to produce carbonate rocks, removing the carbon dioxide (CO2) from the atmosphere. This is partly what keeps CO2 on Earth under control -- plant activity also helps. But Venus, which lost its water, was unable to stop the gradual release of CO2 from volcanos, which eventually built up to form the thick CO2 atmosphere we see today.

All of this was caused by just a little extra sunlight striking Venus than Earth, leading to a chain of giant effects.

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