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).


I. ATMOSPHERE OF THE EARTH

First look at the current atmosphere of the Earth. 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 atmosphere is conveniently divided into regions in terms of how the temperature behaves (whether it is increasing or decreasing):

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

  • 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 Ozone lives. In the stratosphere, because Ozone absorbs Solar ultraviolet radiation, temperature increases as you move upward 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. The troposphere is the layer where weather is generated. In the troposphere, temperature declines with altitude. On average, the temperature declines with height at rate -6.5oC per kilometer in the lower troposphere. At the top of the troposphere, clouds form (because it gets too cold for water to be vapor). This traps water in the troposphere, the so-called Cold Trap. Because the ozone layer lies in the Stratosphere, the water in the troposphere is shielded from the Solar UV radiation and is not destroyed by photodissociaion.



II. ATMOSPHERIC EVOLUTION: EARTH, MARS, VENUS

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.

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 water, carbon dioxide, sulfur dioxide, carbon monoxide, suflur, cholorine, nitrogen, molecular hydrogen, sufur, nitrogen, and chlorine, ammonia, and methane. On each of Venus, Earth and Mars, liquid oceans likely formed initially. On the Earth, oceans formed in the Early Archean period (the time before 2.5 billion years ago perhaps as long ago as 4 billion years). Despite all having started with oceans only the Earth has retained extensive oceans.

What caused the difference in the evolution of the atmospheres of the Terrestrial planets and liquid oceans as shown above?

    On Venus, Earth and Mars, carbon dioxide initially 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 first two processes are less efficient at higher temperatures.) Carbon dioxide deposited into the oceans, settled and formed sedimentary rocks ===> carbon dioxide was trapped in the crust! This happened fairly quickly on Earth:


After this initial start-up, the evolutionary paths of Venus, Earth, and Mars then diverged.



III. WHAT ABOUT THE FREE OXYGEN IN THE EARTH'S ATMOSPHERE?

Today, we see that the atmosphere of the Earth contains ~21 % free oxygen. As noted above, at birth there was no free oxygen. This is good because chemical reactions thought to produce amino acids are inhibited by oxygen Where did the oxygen come from?

Oxygen Production:

    (i) Photochemical dissociation - breakup of water molecules by ultraviolet produced free oxygen at ~ 1-2% levels. At these levels, ozone can form to shield Earth surface from ultraviolet (UV) radiation.

    (ii) Photosynthesis - carbon dioxide + water + sunlight ===> organic compounds + oxygen molecules. Produced by cyanobacteria (photosynthetic prokareyotes--blue-algae), and eventually higher plants supplied the rest of oxygen to the atmosphere.

In the Archean period (4 billion years to 2.5 billion years ago), there was little or no free oxygen in the atmosphere (< 0.001 % of the current level of oxygen, PAL). Around a billion years before the end of the Archaen period, photosynthesis started. The little oxygen produced by cyanobacteria was probably consumed by the weathering process. Only after rocks at the surface were sufficiently oxidized could free oxygen remain free in the atmosphere.

Interestingly, during the Archaen period, the day may have been as short as 12.3 h with a year lasting 714 days. During this time, the Moon slowed the Earth's rotation rate increasing the length of the day. It was recently suggested that this may have led to the jump in oxygen signaling the end of the Archean period. With a longer day, cyanobacteria had a longer time for uninterrupted photosynthesis which increased oxygen production.

During the Proterozoic era (2.5 to 0.5 billion years ago), the free oxygen rose to 1 % to 40 % of PAL. Most of the oxygen was released by cyanobacteria, which showed a strong increase in abundance (in the fossil record) about 2.45 billion years ago. The present level of free oxygen probably was achieved around ~400 million years ago which coincided with a 3-fold increase in biodiversity on the Earth.