The Gaseous Phase Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 5, 2011

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 (see Fig. 10-5b). 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.

Gases In Outer Space

Outer space was once believed to be a perfect vacuum. However, this is not the case. There is plenty of stuff out there, and much of it is hydrogen and helium gas. (There are also trace amounts of heavier gases and some solid rocks and ice chunks as well.) All the atoms in outer space interact gravitationally with all the others. This might be hard to imagine at first, but if you think about it, there’s no escaping it. Even a single atom of hydrogen exerts a gravitational pull on another atom 1 million km away.

The motion of atoms in outer space is almost random but not quite. The slightest perturbation in the randomness of the motion gives gravitation a chance to cause the gas to clump into huge clouds. Once this process begins, it can continue until a globe of gas forms in which the central particle density is significant (see Fig. 10-5c). As gravitation continues to pull the atoms in toward the center, the mutual attraction among the atoms there becomes greater and greater. If the gas cloud has some spin, it flattens into an oblate spherical shape and eventually into a disk with a bulge at the center. A vicious circle ensues, and the density in the central region skyrockets. The gas pressure in the center rises, and this causes it to heat up. Ultimately, it gets so hot that nuclear fusion begins, and a star is born. Similar events among the atoms of the gas on a smaller scale can result in the formation of asteroids, planets, and planetary moons.

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