Summary of the lecture on 07-29-96
The Role of H2O and CO2 in the Evolution of
Venus, Earth, and Mars
This class presented another look at the maybe surprising fact
that in spite of the many similarities between the 3 planets Venus,
Earth, and Mars, they evolved into very different places. The actual
processes that went on during the evolution of the planetary atmospheres
were very complex in detail, but we can understand it in general
just by considering what happened to two molecules:
If these molecusles exist in the atmosphere of a planet, they will
allow most of the visible radiation coming from the sun to penetrate
down to the surface of the planet (they are transparent to visible light).
This radiation will be absorbed by the planet's surface and heat it up.
The planet will then re-radiate this energy (otherwise it would continue
to heat up) in the infrared. H2O and CO2 are opaque
to (the bulk of) this infrared radiation. IR photons get absorbed and re-emitted
many times, so the total amount of energy in the atmosphere is increased -
the atmosphere heats up. This process is often referred
to as the greenhouse effect.
What are the key factors that led to such extremely different results in spite of the similartities of the processes at work?
- Venus is closer to the Sun, therefore CO2 cannot be bound in rocks. Therefore, all the
CO2 is in the atmosphere (the total amount is similar to that of the Earth!!) and the resulting runaway greenhouse effect leads to the loss of
all H2O.
- On Earth, most of the CO2 is bound in rocks and does not contribute to the greenhouse effect. There is a cool atmospheric layer that condenses evaporated water and prevents it from getting past the ozon layer where it could be broken up by the Sun's UV. The average temperature is increased by ~35 K
compared to what it would be without the greenhouse effect.
- Mars's small size allowed the planet to cool off quickly. It does not have
a molten core and therefore no plate tectonics. CO2 once bound in rocks will not be re-released into the atmosphere by volcanic erruptions. The greenhouse effect is very weak and only increases the average temperature by ~6 K. Mars's small size also means that the escape velocity for gas molecules
is smaller. The early atmosphere may have been lost by the"runaway refrigerator" process, but even without this effect, Mars would not have been able to hold on to it.
The Carbon Cycle
On Earth, the bulk of the carbon atoms go through the following (~500,000 year-) cycle. We start we atmospheric CO2.
- rain + CO2 -> H2CO3 "acid rain"
- H2CO3 combines with rocks
- rocks weather/erode and get washed into rivers and oceans
- gets incorporated into "life" (plankton shells -> CaCO3 = clacium carbonate)
- forms sedimatary layer
- pressure of top layers turns it into limestone
- plate tectonics/subduction melts rocks
- CO2 is recycled into atmosphere by volcanic erruptions
Stability of the Climate on Earth
Our large moon stabilizes tilt of Earth's rotation axis which in turn keeps season stable. The slight variations of this tilt combined with slight changes of the ellipticity of the Earth's orbit and the orientation of the orbital ellipse are to a large extent responsible for the ice ages. If the changes were more extreme, as is the case for Mars, the "ground rules" for life would continuously change which would make survival much harder.
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