Gases Near A Planet
Now imagine the shroud of gases that compose the atmosphere of a planet. Gravitation attracts some gas from the surrounding space. Other gases are ejected from the planet's interior during volcanic activity. Still other gases are produced by the biological activities of plants and animals, if the planet harbors life. On the earth, some gases are produced by industrial activity and by the combustion of fossil fuels.
All the gases in the earth's atmosphere tend to diffuse into each other when we look at parcels of reasonable size, regardless of the altitude above the surface. But there is unlimited "outer space" around our planet and only a finite amount of gas near its surface, and the gravitational pull is greater near the surface than far out in space. Because of these factors, diffusion takes place in a different way when considered all the way from the earth's surface up into outer space. The greatest concentration of gas molecules (particle density) occurs near the surface, and it decreases with increasing altitude (Fig. 1-4B). The same is true of the number of kilograms per meter cubed of the atmosphere, that is, the mass density of the gas.
On the large scale of the earth's atmosphere, yet another effect takes place. For a given number of atoms or molecules per meter cubed, some gases are more massive than others. Hydrogen is the least massive. Helium has low mass, too. Oxygen is more massive, and carbon dioxide more massive still. The most massive gases tend to sink toward the surface, while the least massive gases rise up high, and some of their atoms escape into outer space or are not permanently captured by the earth's gravitation.
There are no distinct boundaries, or layers, from one type of gas to another in the atmosphere. Instead, the transitions are gradual. That's good, because if the gases of the atmosphere were stratified in a defined way, we would have no oxygen down here on the surface. Instead, we'd be smothered in some noxious gas such as carbon dioxide or sulfur dioxide. We'd have to climb mountains in order to breathe!
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