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Question
Why do you think Earth and Venus have an atmosphere and Mercury has only trace gases, while Mars a very weak atmosphere?
The ability of a planet to hold on to an atmosphere depends primarily on its size (surface gravity), and secondly on its temperature. There are standard formulas for calculating the escape rates of gas molecules based on surface gravity and temperature. A third factor is the nature of the chemical interactions of the gas with the surface, or even with life. The Earth would have a much thicker atmosphere, like that of Venus, if most of our carbon dioxide were a gas rather than being sequestered in sediments. Since microbes take carbon dioxide out of the atmosphere, our biosphere has played a crucial role in balancing the composition and amount of gas we have in our atmosphere. David Morrison
NAI Senior Scientist
September 22, 2006
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