Earth's Atmosphere

The Earth's atmosphere comtains the very air that we breathe, the rain that makes the flowers grow, molecules that make the sky blue, and chemicals that give us protection from some really harmful forms of every from the Sun. In this section, we will explore the atmosphere of the Earth, and look at other places in the Solar System to see how the atmosphere of Earth compares to that of Venus, Mars, Jupiter, and Saturn's moon Titan.

Earliest Atmosphere

The Earth did not always have the same atmosphere as the one we depend on today. In the earliest atmosphere, the molecules of H2 and He2 dominated. These lightest of the molecules did not stay long. Their are light in molecular weight, and therefore require a large gravitational attraction to keep them. The Earth is about 1/3 too light in terms of mass. Additionally, the Earth was a homogeneous mixture of molten rock and chemicals that had not yet stratified into the layers we know today. Without a iron core, there is not magnetic field in the early Earth, which is required to magnetically hold some of the lightest elements. Constant volcanism and the decay of radioactive elements everywhere kept the planet very hot. With a thin or perhaps no basalt crust, the molten planet made water accumulation impossible.

Next Stage in Atmosphere Development

Later, after the Iron Catastrophe stratified the interior of the Earth, and a magnetic field formed to hang on to gases and protect the surface from ultraviolet radiation, the atmosphere filled up with carbon dioxide and water vapor venting from the rock via volcanism. There were smaller amounts of hydrogen, helium, methane and ammonia. A question remains among early-Earth scientists as to the origin of the oceans, but this is another lesson. As the oceans grew in size, more and more carbon dioxide was dissolved into the water. This process is discussed in greater detail in the section on the Carbon Cycle. More carbon dioxide combined with compounds to form silicon dioxide, limestone, calcium carbonate and other mineral sediments. This began to leave the atmosphere rich in nitrogen.

The removal of all that carbon dioxide was an important step in Earth's history. Too much carbon dioxide in the atmosphere will trap heat by a process called the greenhouse effect. Sunlight coming in warms the surface. Heat then radiates from the surface as infrared radiation. But carbon dioxide will not allow infrared radiation to escape and so the planet continues to heat up with no way for the heat to escape. This is taught in greater detail in Global Warming. Ultraviolet radiation from the Sun removed the methane and ammonia. What is fascinating is the measurements of the rate of nuclear fusion in the Sun. Early on, the Sun had a less dense and much cooler core temperature. As helium was made, the core of the Sun shrunk and the internal temperatures began to increase. An increasingly hot Sun would certainly prove disastrous to an atmopshere with a lot ofcarbon dioxide. On the other hand, a cool Sun would put out less radiation and make the planet cold. It is believed by planetary scientists that the Sun was 30% more dim in the distant past. However, the early Earth carbon dioxide levels were much greater. This resulted in the Sun's heat being trapped and raising global temperatures. As the Sun heated up, the global levels of carbon dioxide decreased. The result seems to indicate that at no time has the Earth suffered extreme temperatures that would result in complete boiling of the oceans or complete freezing, Ice Ages notwithstanding. In time, the Sun will continue to heat up, and the Earth will eventually succumb, which is a topic you can check out in an article entitled Earth Dies.

Life Adds Oxygen

When the period of bombardment by large rocks from space ended about 3.8 billion years ago, the planet's surface stabilized. According to the fossil record, lifeforms appeared on the planet about 3.5 billion years ago. These earliest bacteria appear to have gotten their eneregy from geothermal vents in ocean crust cracks, and their carbon from carbon dioxide dissolved in the water. At some point, unsure as to when, a life form appeared on the Earth that released oxygen as a by-product of metabolism. The very oxygen that we breathe in was a terrible poison to the early lifeforms because it is such an electronnegative atom. The result of oxygen release from early Earth organisms was also the death of many of these organisms. Photochemical reaction of solar energy with water vapor dissociates the H2O, freeing a small amount of gaseous oxygen (estimated at 1-2% of current levels). With a little ligntning in the atmosphere, ozone is made from O2 into O3. With ozone, there now exists a protective atmospherive barrier to life-threating ultraviolet radiation as well as the poisonous oxygen. Finally, about 2.1 billion years ago, oxygen began to accumulate in the Earth atmosphere. The thinking is that a lifeform evolved that could utilize the electronegtivity of oxygen as a force to pull high energy electrons. With the advent of electron transport in blue-green algae, and other bacteria like it, the exchange between carbon dioxide and oxygen in the air began in a cyclic manner. Oxygen levels continued to increase, and lifeforms became dependent on this element to drive metabolic processes that make ATP. Even early on, the recycling of compounds between the abiotic (non-living) and biotic (living) world was happening.

Excerpt from the University of Michigan course on Global Change

Life started to have a major impact on the environment once photosynthetic organisms evolved. These organisms, blue-green algae (picture of stromatolite, which is the rock formed by these algae seen lower left), fed off atmospheric carbon dioxide and converted much of it into marine sediments consisting of the shells of sea creatures. While photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to produce oxygen. For a long time, the oxygen produced did not build up in the atmosphere, since it was taken up by rocks, as recorded in Banded Iron Formations (BIFs; picture seen lower right) and continental red beds. To this day, the majority of oxygen produced over time is locked up in the ancient "banded rock" and "red bed" formations. It was not until probably only 1 billion years ago that the reservoirs of oxidizable rock became saturated and the free oxygen stayed in the air. Once oxygen had been produced, ultraviolet light split the molecules, producing the ozone UV shield as a by-product. Only at this point did life move out of the oceans and respiration evolved.

The Early atmosphere was probably dominated at first by water vapor, which, as the temperature dropped, would rain out and form the oceans. This would have been a deluge of truly global proportions an resulted in further reduction of CO2. Then the atmosphere was dominated by nitrogen, but there was certainly no oxygen in the early atmosphere. The dominance of Banded-Iron Formations (BIFs; see picture above) before 2.5Ga indicates that Fe occurred in its reduced state (Fe2+). Whereas reduced Fe is much more soluble than oxidized Fe (Fe3+), it rapidly oxidizes during transport. However, the dissolved O in early oceans reacted with Fe to form Fe-oxide in BIFs. As soon as sufficient O entered the atmosphere, Fe takes the oxidized state and is no longer soluble. The first occurrence of redbeds, a sediments that contains oxidized iron, marks this major transition in Earth's atmosphere.

More details on the Red Beds and Banded Iron Formations are found at the Columbia's website on the Early Evolution of Earth.

Today's Atmosphere

Our current atmosphere consists of about 79% nitrogen, 20% oxygen and tiny amounts of water vapor, carbon dioxide, ozone and argon. The atmosphere reaches about 150 km above our planet. It also thins as elevation increases, humans have a difficult time breathing and functioning at high elevations without oxygen support.

The overall composition of the earth's atmosphere is summarized below along with a comparison to the atmospheres on Venus and Mars - our closest neighbors.

Atmospheric Comparison

Why is the atmosphere of Venus (Earth's sister) so different from Earth?

On Earth, CO2 is absorbed in the oceans and rocks. If the absorbed CO2 on Earth were released into the atmosphere, 98% would be CO2 and the atmospheric pressure would be 70x what it is now. So, except for the O2 and water, Earth's atmosphere would be similar to Venus' if the CO2 has not gotten absorbed. The oxygen on Earth is a product of life, the result of photosynthetic splitting of water into O2 and H+ ions that are used to drive the future synthesis of ATP and Glucose.
The water on Venus has disappeared due to the extreme temperatures brought on by the runaway greenhouse effect. The trapped radiant heat from the solar-heated ground caused water vapor to rise to high elevations where is was then broken down into oxygen and hydrogen by ultraviolet radiation (photolysis). The hydrogen, being light, escaped from the gravitational control of Venus. The remaining heavier oxygen radicles combined with other atmospheric gases, and thus water on Venus was lost forever!

Summary of Atmospheric Components

Evolution of Venus' Atmosphere

During secondary atmosphere formation on Venus, the temperature was so high that no oceans formed. (Water is broken down by a process called photolysis, and H2 escapes) and gases are not absorbed back into the rock as they are on Earth. The remaining carbon dioxide gases in the atmosphere (pumped out via volcanism) trapped heat (greenhouse effect). As the temperature rose even more, the planet found itself in a runaway greenhouse effect. The atmospheric heating was unstoppable. Given the closer proximity to the Sun and the extremely slow rotational speed of the planet, Venus was doomed to be lifeless. What early planetary astronomers believed would be a world teeming with life turned out to be a dry, barren, and exceedingly harsh environment, and perhaps the worst surface place in the Solar System to visit.

The variations in concentration from the Earth to Mars and Venus result from the different processes that influenced the development of each atmosphere. While Venus is too warm and Mars is too cold for liquid water the Earth is at just such a distance from the Sun that water was able to form in all three phases, gaseous, liquid and solid. Through condensation the water vapor in our atmosphere was removed over time to form the oceans. Additionally, because carbon dioxide is slightly soluble in water it too was removed slowly from the atmosphere leaving the relatively scarce but unreactive nitrogen to build up to the 78% is holds today.

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