EVOLUTION OF THE ATMOSPHERES OF THE TERRESTRIAL PLANETS

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

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, the greenhouse gases water, carbon dioxide, methane, and CFCs.

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,

  • 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).

  • 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.

  • 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.

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

What Happened to Venus and Mars?

The Terrestrial planets (the atmosphere ones) are roughly the same sizes and same distances from the Sun and yet, they have grossly different kinds of atmospheres and conditions on their surfaces. Do we have any ideas as to what leads to the huge differences? Surprisingly, there may be simple explanations.

Venus, Earth, and Mars

In the beginning, we believe that the material which was outgassed from the interiors or carried in by comets onto the Terrestrial planets was similar. That is, the Terrestrial planets started out roughly the same. Originally dominated by carbon dioxide, water, carbon monoxide, ... (and perhaps methane and ammonia). On the Earth and Mars, we had,

  • On Earth and Mars, the carbon dioxide dissolved into the oceans, was rained out of the atmosphere (and then washed into the oceans), or was directly adsorded into the rocks and washed into the oceans. The carbon dioxide deposited into the oceans, settled and formed sedimentary rocks ===> the carbon dioxide got trapped in the crust! On the Earth, volcanism (and plate tectonics) returns a little carbon dioxide to the atmosphere (to keep our Greenhouse effect going).

  • On Mars, because there was no large scale plate tectonics, the carbon dioxide was only taken out of the atmosphere; it was not recycled back. This caused the Greenhouse effect to go away. The oceans then froze and/or much of the free water and whatever was left of the carbon dioxide rose to high levels in the atmosphere where they were dissociated and lost to space. In either the water was lost. This process is quick in that it can occur in less than around 1 billion years.

On Venus, it is believed that it was too warm (because Venus is 30 % closer to the Sun than is the Earth) for there to be extensive liquid oceans. This meant that the carbon dioxide would remain in the atmosphere. Further, because water vapor is also a good Greenhouse gas, the early temperature of Venus may have reached 2,700 F and the surface pressure may have been 300 bars (or the pressure one would feel living at a depth of 3 km under the ocean). This is not fun.

The water vapor was then free to rise up into the high levels of Venus's atmosphere where it was broken up by solar radiation and the hydrogen from the water escaped to space. This caused Venus to lose its water. After the water was lost, the Greenhouse effect eased and the temperature dropped to ~ 900 F and the pressure dropped to 100 bars.

THE GAIA HYPOTHESIS

MARS

An upshot of the above scenario is that in the past Mars could have had a much thicker atmosphere and been much more earth-like (there are models which suggest that the early Mars had an atmospheric pressure of 2 bars!). This is interesting because today, the atmospheric conditions on Mars are such that liquid water cannot exist on the surface of Mars. However, there is water because we see evidence of it in the Martian polar ice cap. The polar caps on Mars have two parts; a part which shows seasonal variations and residual caps. The seasonal caps are thought to be composed of frozen carbon dioxide. The residual caps are smaller and brighter than the seasonal caps and show a very marked north-south asymmetry. The southern residual cap is frozen carbon dioxide while it is believed that the northern cap is water ice (supported by the observation that water vapor is observed over the residual cap in the northern summer).

There, presumably, is also a permafrost layer on Mars even today as implied by Outflow Channels, "Islands", and Splosh Craters. These features were produced by massive floods on Mars. Presumably what happened was that some event caused a large-scale rapid melting of the permafrost layer which caused floods.

There is also evidence that in the past that water existed in liquid form on the surface of Mars ===> grossly different atmosphere in the past than today. There is thus a large body of evidence that the climate of Mars may have been more Earth-like in the past than it is today. This leads to the hope that perhaps life existed on Mars in the past.