ATMOSPHERES OF THE TERRESTRIAL PLANETS

We now address the atmosphere of the Earth. We consider:

• the current atmosphere of the Earth; and

• the evolution of the atmosphere of the Earth.

After this, we consider the atmospheres of Venus and Mars (and address the question of why the atmospheres of the three planets are so different).

Atmosphere of the Earth

We first look at the current atmosphere of the Earth. Recall that the current atmopshere of the Earth has a pressure of 1 bar which is ~ 100 times larger than Mars and ~ 1 % that of Venus. The composisiton of the Earth's atmosphere is 78 % Nitrogen molecules ad 21 % Oxygen molecules with trace amounts of of other things, in particular, water, carbon dioxide, methane, and CFCs (more on these things later).

The pressure in the atmosphere declines as you move up in altitude (Why?) and is conveniently divided into regions in terms of how the temperature behaves,

• Troposphere: The lowest layer of the atmosphere. The troposphere contains most of the water and consequently, is the layer in the atmosphere where most of the weather is generated. In this layer, the temperature declines as you move up in altitude.

• Stratosphere: The next higher layer of the atmosphere is known as the Stratosphere and is broken up into layers of different materials (i.e., it is stratified from which follows its name). The stratosphere is the layer where the Ozone lives. In the stratosphere, because the Ozone abosrbs the solar ultraviolet radiation, the temperature increases as you move up in altitude in the stratosphere.

• Mesosphere: Nothing of real interest goes on here. There are no real strong absorbers of solar radiation and so the temperature decreases in this layer as you move up in altitude.

• Thermosphere (or Ionosphere): In this layer, the solar radiation is able to ionize (strip electrons off of atoms) and thus the temperature again starts to rise (since the atoms again absorb solar radiation). This is the layer which traps radio signals and allows them to be heard around the world (it is also the layer which gets disturbed and disrupts radio communication).

All of the Terrestrial planets have atmospheres which show one or more of these regions depending their make-ups.

Now let us return to the question of why the Earth's atmosphere is warmer than suggested by simple equilibrium arguments. To understand this point, we must consider the,

Greenhouse Effect

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