MadSci Network: Earth Sciences
Query:

Re: can the green house effect on venus be reversed or carbon removed

Date: Tue Feb 3 10:35:55 2004
Posted By: Carolyn Ernst, Grad student, Planetary Geology
Area of science: Earth Sciences
ID: 1075148370.Es
Message:

Hi, Jack.  The quick answer to your question is no, not presently.  Venus itself
does not have the ability to remove the carbon dioxide from its atmosphere, and
humans do not have the capability to remove it.  A longer answer is provided below.

For a description of how the greenhouse effect may have developed on Venus, see
this previous MadSci answer.  

The greenhouse effect is caused by Venus' atmosphere.  Currently, carbon dioxide
(CO2) is the gas in the atmosphere that maintains Venus' greenhouse effect. 
Most light the sun shines on Venus is visible light.  This light is able to
scatter through Venusís atmosphere and reach the surface, which heats up the
planet.  Venus re-radiates this energy as infrared (IR) light, which has a
longer wavelength than visible light.  The atmosphere is not transparent to IR
light (it is absorbed by CO2), so Venus cannot efficiently cool itself.  Heat 
builds up and warms the planet to much higher temperatures than if the
greenhouse effect did not exist.  

The Earth also experiences a mild greenhouse effect.  One major difference
between the planets is that the major constituent of Earthís atmosphere is
nitrogen, which is not a greenhouse gas.  Earth has several wavelength ranges
where IR light can escape the planet.  The major constituent of Venusís
atmosphere is CO2, which is a greenhouse gas.  

Both planets contain about the same amount of CO2 but it is located in different
places.  Venusís CO2 is mostly found in the atmosphere, which is 96% CO2 and has
an atmospheric pressure approximately 90 times that of Earth (90 bars versus 1
bar).  Earthís atmosphere is only about 0.03% CO2.  Most of the remaining CO2 is
trapped in carbonaceous rocks, such as limestone.  These rocks are made by the
reaction of water and CO2 with silicate rocks.  Most of the resulting product is
washed into the oceans and is deposited as sedimentary rocks.  The oceans
themselves also absorb CO2 directly from the atmosphere.  Plants use 
water to convert CO2 into oxygen and sugar through photosynthesis.  Clearly,
water is a very important factor in controlling the amount of CO2 that remains
in the atmosphere and thus the significance of the greenhouse effect.

As described in the greenhouse effect reference above, Venus has lost most of
its water.  Ultraviolet light from the sun broke up the water into its
constituent parts, hydrogen and oxygen.  The hydrogen escaped from the planet
and the oxygen reacted with surface rocks, removing it from the atmosphere.  

There is no source that can provide Venus with more water.  Even if there were,
the present surface environment cannot support liquid water.  It is also much
too warm and has too high an atmospheric pressure to support life. Because of
this, Venus cannot possibly remove CO2 from its atmosphere in the same way that
Earth does.  

It is not presently feasible for humans to physically remove the CO2 atmosphere
of Venus.  Even if it were possible for a spaceship to scoop out some of the
atmosphere, it would take a very very long time to remove all of the CO2.  

In an imaginary scenario, if it were possible to remove 89 of the 90 bars of
atmosphere, Venus would be left with the same atmospheric pressure as the Earth,
but 3200 times the amount of CO2.  Earthís atmosphere contains only trace
amounts of greenhouse gases, so Earthís greenhouse effect is small compared to
that of Venus, but it raises the average temperature by approximately 60 degrees
Fahrenheit (~33 degrees Celsius).  Thus, 3200 times the amount of CO2 would
still have quite a significant greenhouse effect on Venus.  





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