Modern atmospheric composition: as discussed last time, it is mostly nitrogen, oxygen, and argon. "Modern" means within about the last 600 million years of Earth's 4.6 billion-year-old history. Compared to the primordial atmosphere, this composition is completely different. What happened? You must consider:
With these questions in mind, some of the factors in the evolution of the atmosphere are:
The present atmosphere is primarily the result of volcanic outgassing, though. Some evidence: the composition of gases from volcanoes today is similar to the composition of atmospheric gases. The argon:nitrogen ratio in volcanoes and the atmosphere is the same. However, volcanoes put out much more carbon dioxide and H2O compared to what we see in the atmosphere. Where does all this extra CO2 and H2O go? H2O is taken up by the oceans. CO2 is changed to CaCO3 in the oceans and is used up in photosynthesis as well.
80-480 km: thermosphere (top is "thermopause")
50-80 km: mesosphere (top is "mesopause")
18-50 km: stratosphere (top is the "stratopause")
0-18 km: troposphere (top is the "tropopause")
Why is weather found in the troposphere and not the stratosphere?
Well, if we have some air that is warm at the bottom and cool at the top, the cool air is going to want to sink and the hot air is going to want to rise. The result is turbulent mixing, or "convection." This is the situation in the troposphere. If we look at the stratosphere, we see temperature is colder on the bottom and hotter on the top. This is stable because cold air naturally wants to be at the bottom, and so the stratosphere will not want to mix itself. This stable situation is called an inversion.
Why does the atmosphere have 2 inversions?
The first inversion in the stratosphere is due to ozone. Ozone absorbs UV radiation, which causes it to heat up. (More UV is soaked up at the top, so the top becomes hotter.)
The second inversion in the thermosphere is also caused by absorption, but of even shorter wavelength radiation (higher energy). The wavelength of energy absorbed depends also on the size of the molecule doing the absorbing.
The top part of the atmosphere is also called the magnetosphere because it is the region where charged particles in the solar wind interact with Earth's magnetic field (remember the Northern & Southern lights?). This illustrates that using temperature to classify the atmospheric structure is just one way to do it. Other methods are based on the purpose of the classification, such as satellite or radio technology.
Back to GS 201 homepage