Summary of the sections on 06-18-96

Runaway Greenhouse Effect

To understand how Venus's atmosphere became the way it is, consider moving the Earth closer to the Sun to the distance of Venus. The following would happen:
  1. land and oceans warm up
  2. more water vapor gets into the atmosphere; more C02 is "baked" out of the rocks
  3. greenhouse effect becomes stronger
  4. land and oceans warm up even more
  5. ...
At some point, water molecules can rise high enough in the atmosphere that they can be broken apart by ultraviolet radiation from the Sun. Hydrogen escapes the atmosphere and the remaining oxygen reacts with other elements. Strong evidence that this process really occured is the observation that the abundance of HDO (you were right, Carl!) relative to H2O is increased by a factor 100 in Venus's atmosphere relative to the ratio observed on Earth.

Runaway Refrigerator Effect

There is evidence for Mars having had a much denser atmosphere in the past: There are channels that look like dried river beds on Earth. It is very likely that these were formed by liquid water which can only exist at atmospheric pressures much higher that the present value. Mars probably lost its atmosphere by another runaway process which one might call the runaway refrigerator:
  1. amount of CO2 in the atmosphere is reduced by formation of CO2 ice and by disolving CO2 in liquid water.
  2. greenhouse effect becomes weaker
  3. atmosphere cools
  4. amount of water vapor and CO2 is further reduced
  5. greenhouse effect becomes even weaker
  6. ...
Whether this runaway effect really occured on Mars is less certain than in the case of Venus. Other factors may have been very important as well: The small size of Mars, large meteorite impacts, ...

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