Part 5D:

EVOLUTION OF THE ATMOSPHERES OF THE TERRESTRIAL PLANETS

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 atmosphere of the Earth has a pressure of 1 bar which is ~ 100 times larger than Mars and ~ 1 % that of Venus. The composition of the Earth's atmosphere is 78 % Nitrogen molecules ad 21 % Oxygen molecules with trace amounts 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?). The atmosphere is conveniently divided into regions in terms of how the temperature behaves (whether it is increasing or decreasing),

  • Thermosphere: In the thermosphere, the solar radiation is able to ionize (strip electrons off of atoms forming the ionosphere ) and the temperature increases with altitude (because the atoms absorb solar radiation). The ionosphere 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 during Solar storms).

  • Mesosphere: There are no strong absorbers of solar radiation in the mesosphere so the temperature decreases with altitude here.

  • Stratosphere: The next layer of the atmosphere is known as the Stratosphere and is broken up into layers composed 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 absorbs the solar ultraviolet radiation, the temperature increases as you move up in altitude through the stratosphere.

  • Troposphere: The lowest layer of the atmosphere, the troposphere is where atmospheric convection occurs and is the layer which contains most of the water. Consequently, the troposphere is the layer where weather is generated. In the troposphere, the temperature declines with 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, they were 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 ( which is prevented on the Earth by the rising temperature in the stratosphere produced by the UV absorption in the Ozone layer. This hot layer, in effect, puts a lid on the lower lying water vapor forming a water trap). At the high altitudes, the carbon dioxide and water were broken apart by Solar Ultraviolet radiation and lost to space. The above process is quick; it can occur in less than a billion years.

On Venus, it is believed that it was too warm for there to be extensive liquid oceans and the water remained in the atmosphere (Venus is roughly 30 % closer to the Sun than is the Earth and receives around twice as much Solar energy). This meant that all of the carbon dioxide remained in the atmosphere and a Runaway Greenhouse Effect ensued. Furthermore, 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.

Where is Venus's Water?

Because there is no Ozone layer, the temperature simply decreases as you move up in altitude around Venus. There is not a water trap and the water vapor is free to rise up into the high levels of Venus's atmosphere where it is broken up by Solar radiation. The hydrogen atoms from the water then escape to space and the oxygens combine with other atmospheric gases to form different molecules. Venus thus loses its water. After the water is lost, the Greenhouse effect eases and the temperature drops to the mild ~ 800-900 F of today and the pressure drops to 90 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. We do see evidence, however, for water on Mars. For example, there is in water in the northern residual polar ice cap, permafrost layers, splosh craters, ... .

The polar caps on Mars have two parts; regions that show 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 residual cap is water ice (supported by the observation that water vapor is observed over the residual cap in the northern summer and the temperatures of the caps).

There, presumably, is also a permafrost layer on Mars even today as implied by Outflow Channels, "Islands", and Splosh Craters. The outflow channels and islands were produced by massive floods on Mars. Presumably what happened was that some event (possibly the impact of a large object) caused a rapid, large-scale melting of the permafrost layer which caused floods.

There is also evidence that in the past water existed in liquid form on the surface of Mars ===> grossly different atmospheric conditions in the past than presently. There is thus 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.