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 free Oxygen is unusual as neither Venus nor Mars have it. On the Earth, the free Oxygen is a result of life.

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), The behavior of the temperature is another matter, however.

  • 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 carrying heat from the Earth' surface upward and is the layer which contains most of the water and atmosphere (75 % of the mass of the atmosphere is contained in the 10-15 kilometers of the troposphere). The convection drives air currents, and generates turbulence; the troposphere is the layer where weather is produced on the Earth. In the troposphere, the temperature declines with altitude.

At the top of the troposphere, the the tropopause, water vapor turns to ice and the ice chunks cannot continue to rise in altitude. In this manner, water is trapped in the troposphere, this is the so-called Cold Trap. If the water vapor had continued to rise, then it would have eventually been subjected to the ultraviolet (UV) radiation from the Sun which would have dissociated it;

H2O + Ultraviolet ----> H + OH

The hydrogens would then escape, and the oxygens woulc be taken up by the crust and through the formation of other molecule ---> the water would be destroyed (lost)!

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, 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 water in the oceans froze, and some of the free water vapor in the atmosphere and whatever was left of the carbon dioxide, froze and/or rose to high levels in the atmosphere where they were photodissociated and lost. The loss process is quick; it can occur in a few hundreds of millions of years, certainly 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 is likely true even though the Sun was fainter in the past, The Faint Young Sun Problem). 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?

In the past when there was water vapor in Venus's atmosphere, the temperature in the atmosphere was very high. In this environment, the water vapor would rise to high altitudes in the atmosphere where the UV radiation from the Sun is free to photodissociate the water molecules; the water is thus broken up and the hydrogen atoms 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 CO2 atmosphere, a strong Greenhouse Effect, and been much more Earth-like, despite the Faint Young Sun problem. (There are, in fact, models which suggest that the early Mars had an atmospheric pressure of 2 bars!). This is interesting because, today, the atmospheric conditions on Mars ( the very low atmospheric pressure which means that water boils at very low temperatures) 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 water in the northern and southern residual polar ice caps, 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 because at the low atmospheric pressures on Mars, carbon dioxide CO2 melts at temperatures of -125 C while water ice melts at 0 C (interestingly, for the atmospheric pressure on Mars, water is near its Triple Point. The residual ice caps are smaller and brighter than the seasonal caps and show a very marked north-south asymmetry because of the difference in altitudes of the northern and southern hemispheres on Mars. It is used to be thought that the southern residual cap was mainly frozen carbon dioxide while the northern residual ice cap was water ice. More recent data suggests that the southern polar ice cap also contains a large amount of water ice.

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.

The above features do not suggest quiescent water flows. They arise from impacts or other forms of catastrophic heating. There is also evidence that, in the past, water existed in equilibrium liquid form (run-off channels, sinuous river-beds, ...) 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 although we have not yet found life on Mars ( Viking experiments, and somewhat ambiguous results), perhaps life existed on Mars in the past.