GASES NEAR A PLANET

Now imagine the gaseous shroud that surrounds a reasonably large planet, such as our own Earth. 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 biologic activities of plants and animals, if the planet harbors life. In the case of 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, but because there is an essentially unlimited amount of “outer space” and only a finite amount of gas, and because the gravitational pull of the Earth is greater near the surface than far out in space, the diffusion takes place in a different way than inside a small container. The greatest concentration of gas molecules (particle density) occurs near the surface, and it decreases with increasing altitude. 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 is light too. Oxygen is more massive, and carbon dioxide is more massive still. The most massive gases tend to sink toward the surface, whereas the least massive gases rise up high, and some of their atoms escape into outer space or are not captured permanently 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 and vague. This is 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.