Summary of the lecture on 07-22-96
A Comparison of Venus, Earth, and Mars
Venus - Some Facts:
- atmospheric pressure: ~100 times that of the Earth
- distance from Sun: 0.7 A.U.
- surface temperature: 750 K
- same mass and radius as the Earth
- atmospheric composition: 96% CO2
- evidence for (probably still active) volcanism
Earth - Some Facts:
- average surface temperature: 290 K
- atmospheric composition: 78% nitrogen, 21% oxygen, 0.03% CO2
- water exists in gaseous, liquid, and solid phase
- surface is young due to erosion
- plant and animal life
Mars - Some Facts:
- temperature varies strongly between day time and night time
- atmospheric pressure: 0.01 atm
- atmospheric composition: ~100% CO2
- mass is 10% of mass of Earth, radius is 40% of radius of Earth
- distance from Sun: 1.5 A.U.
- no liquid water because of low atmospheric pressure, but evidence for its existance in the past
- (probably) extinct volcanism
Why did these planets become so different from each other dispite the fact that they are at similar distances from the Sun?
In equilibrium, the amount of energy received from the Sun in the form of visible light is the same as the amount of enrgy radiated away in the IR. The temperature adjusts itself to achieve this balance (its actually a bit higher for the Earth and a lot higher for Venus due to the greenhouse effect, which will be discussed next time). The different types of molecules in the atmosphere all have the same averege energy. This means that the lighter ones move faster. If they move faster than the escape velocity
of the Earth (about 11 km/s on the surface), they will escape.
(more on this next time)
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