Part 5D: EVOLUTION OF THE ATMOSPHERES OF THE TERRESTRIAL PLANETS

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 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. At the top of the troposphere, clouds form (because it gets too cold for water to be vapor). This locks the 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.

What Happened to Venus and Mars?

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 water, carbon dioxide, sulfur dioxide, carbon monoxide, suflur, cholorine, nitrogen, molecular hydrogen, sufur, nitrogen, and chlorine, ammonia, and methane.As the Earth evolved and cooled liquid water was able to form. There is evidence that there was liquid water as long as 4.2 billion years ago and that it was never so hot that liquid water could not exist. Extensive oceans clearly formed by early in the Early Archean period (the time between 4 billion to 2.5 billion years ago) as the Earth cooled. What are the consequences of the formation of extensive liquid oceans?
On the Earth and Mars, water vapor likely dominated the early atmosphere, but because of the temperatures, liquid ocean formed:

On Earth and possibly Mars, carbon dioxide combined with water to form carbonic acid in the atmosphere which was rained out of the atmosphere forming calcium, magnesium, potassium or sodium ions on the urfae which were then washed into the oceans; or the carbon dioxide was directly adsorded into the rocks and washed into the oceans. Carbon dioxide deposited into the oceans, settled and formed sedimentary rocks ===> carbon dioxide was trapped in the crust! This happens fairly quickly:

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, 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 rising temperatures 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 the cold trap). At the high altitudes, carbon dioxide and water are broken apart by Solar Ultraviolet radiation. The hydrogen is lost to space destroying the water. This process is quick; it takes less than one billion years. Alternatively, even without the lack of plate tectonics, the Martian atmosphere could be lost from the top down because of Solar UV, and an intense Solar wind. Recent results by Curiosity suggest that the Martian atmosphere was lost from top down.

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.

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 and following years (pre-2.5 billion years ago to 1.8 billion years ago), there was little or no free oxygen in the atmosphere (< 0.001 % of the current level of oxygen, PAL). What little oxygen produced by cyanobacteria was probably consumed by weathering processes and stored in surface rocks. It will only be after rocks at the surface are sufficiently oxidized that could free oxygen remain free in the atmosphere.
During the Proterozoic era (1.8 to 0.5 billion years ago), the free oxygen rose slightly to 1 % to 40 % of PAL as the rocks became less efficient at taking up free oxygen. Most of the oxygen was released by cyanobacteria, as evidenced by an increase in abundance (in the fossil record) about 2.45 to 1.8 billion years ago. Around 400 million years ago, the crust was no filled with oxygen and could no longer take up the released free oxygen; the present level of free oxygen probably was achieved around this time, ~400 million years ago.

Evidence from the Rock Record: (i) iron is extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record, we can infer much about atmospheric evolution. In the Archean period, we find minerals in sediments that can only form in non-oxidizing environments: Pyrite (Fool's gold), and Uraninite. These minerals are easily dissolved out of rocks under present atmospheric conditions. Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert. Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide. BIF's are a major source of iron ore, because they contain magnetite which has a higher iron-to-oxygen ratio than hematite. These are common in rocks 2.0 - 2.8 billion years old, but do not form today. Red beds (continental siliciclastic deposits) are never found in rocks older than 2.3 Billion years, but are common during the Phanerozoic time. Red beds are red because of the highly oxidized mineral hematite that probably forms by oxidation of other Fe minerals that have accumulated in the sediment.
Conclusion - amount of free oxygen in the atmosphere has increased with time.
Biological Evidence: Chemical building blocks of life could not have formed in the presence of free oxygen. Chemical reactions that yield amino acids are inhibited by the presence of even very small amounts of oxygen. Oxygen also prevents the growth of many primitive bacteria such as photosynthetic bacteria, methane-producing bacteria and bacteria that derive energy from fermentation.
Conclusion - Today, most primitive life forms are anaerobic suggesting that the first forms of cellular life probably also had similar metabolism. Today, such anaerobic life forms are restricted to anoxic (low oxygen) habitats such as swamps, ponds, and lagoons. Atmospheric oxygen built up in the early history of the Earth as the waste product of photosynthetic organisms and by burial of organic matter away from surficial decay. This history is documented by the geologic preservation of oxygen-sensitive minerals, deposition banded iron formations, and development of continental "red beds" or BIFs.

THE GAIA HYPOTHESIS

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.

MARS AND WATER
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 water in the northern residual polar ice cap:

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).

Polar Caps and Clouds

In addition to the water in the northern residual polar caps, there is also evidence for water in the low-lying clouds above canyons, and in large glaciers lying scattered rocky debris:

Clouds Above Canyons

Glaciers on Mars

Huge glaciers up to half a mile thick which lie close to the equator of Mars are thought to be the remnants of an ice age on Mars. It is thought that the glaciers formed up to 100 million years ago and represent evidence of climate change on Mars. Hundreds of glaciers have been identified by researchers using ground-penetrating radar which allows them to see through the rocky layers of debris covering the ice. The largest glacier is 13 miles long and more than 60 miles wide. It could be a source of water for astronauts on Mars. When the glaciers formed, Mars' climate was much colder because the angle Mars' spin axis makes with its orbitl axis was much greater than it is now. This allowed ice sheets to extend far beyond the polar regions and towards, possibly even reaching, the Equator.

There, presumably, is also a permafrost layer on Mars even today as implied by Outflow Channels (large channels which can be up to 100 kilometers and thousands of kilometers long--likely formed by catastrophic flooding), "Islands", and Splosh Craters (oozing mud formed by impacts which melted the permafrost layer). 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.
These is ample evidence that water exists on Mars much of it below the surface which can be melted and lead to transient flows.
There is also evidence that in the past water existed in liquid form on the surface of Mars under quiescent conditions ===> 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.

Life on Mars?

The fascination with Mars and life led to attempts to find life on Mars. The first serious attempt was with the Viking landers launced in the 1970s. There were dedicated biological experiments on board designed to search for Law As We Know It (LAWKI). That is, the Viking experiments were designed to detect life based on the example we know best, that is, life on Earth. Is this sensible?

More recent missions to Mars have greatly improved our picture of the Martian surface and the conditions on Mars today. Curiosity was launched on Nov 26, 2011 reaching Mars on Aug 5, 2012 (landing in Gale crater).

A tantalizing (but inconclusive) result was published in the 1990s. A Martian meteorite was discovered on Earth in the Allan Hills of Antarctica. Meteorite ALH 84001 was argued to bear evidence of past life on Mars: