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Earth’s Early Atmospheres and the Development of an Oxygen-Rich Atmosphere

I.	Introduction

While the timing of planetary formation of the Earth is generally well-understood, much debate still surrounds the formation and basic composition of Earth’s early atmospheres. When Earth first formed, most of the gases that would make up the earliest atmosphere were either trapped in Earth’s gravitational field from the planetary nebula or were released from Earth’s rapidly cooling crust. Over the next few billion years, Earth’s atmosphere would undergo drastic changes in its composition that would allow the planet to remain above the freezing temperature of water, permit life of all types to develop, and finally become oxidized to the point we know it today.

II. Earth’s First Atmosphere

Shortly after the formation of Earth, the planet’s atmosphere was very thin or more likely non-existent. Debate continues as to how much of Earth’s early atmosphere was made up of hydrogen and helium. One theory is that residual amounts of hydrogen and helium were trapped within the Earth’s mantle in small quantities and were then outgassed due to tectonic activity. The opposing theory maintains that gases from the planetary nebula were captured in Earth’s gravitational field, and thus hydrogen and helium comprised nearly 30% of the first atmosphere. Proponents of both theories, however, agree that most of the hydrogen and helium gases in the first atmosphere were too energetic to be held by Earth’s gravity for long. Hydrogen and helium have two methods by which they can escape Earth’s gravity: thermal escape and loss by electric fields. Thermal escape, in which hydrogen and helium are heated to levels energetic enough to escape Earth’s gravity, causes most of the loss. However, electrical fields created in the ionosphere can also cause hydrogen and helium to be removed from the atmosphere, although in less abundant quantities. The rate at which hydrogen and helium escaped Earth’s atmosphere is a contentious issue, but most research agrees that helium and hydrogen gases in Earth’s first atmosphere were eventually lost so that only a small fraction remained in Earth’s later atmospheres.

III. Earth’s Second Atmosphere

Earth’s second atmosphere came to gradually replace Earth’s first atmosphere as volcanic activity and outgassing caused a change in gas concentrations. The second atmosphere was mostly made up of compounds that more closely resemble our present atmosphere rather than the light element rich first atmosphere. Earth’s crust, which was filled with methane, ammonia, carbon dioxide, nitrogen, and water, was still solidifying and moving across the planet. As plate tectonics continued, volcanoes on the planet’s surface released large quantities of these gases, which were then trapped in Earth’s gravitational field and built up in the atmosphere. The exact concentration of each gas is still highly controversial, especially concerning the amount of methane and carbon dioxide in the atmosphere.

IV. Faint Young Sun Paradox

Perhaps the biggest unknown in Earth’s first and second atmospheres, concerns the faint young Sun paradox. The formation of Earth occurred approximately 4.5 billion years ago when the Sun was smaller and 30% less luminous. With such a reduction in solar radiation, Earth should have had a temperature of 245K, far below the freezing point of water, but it is known that liquid oceans were present back at least 3.5 billion years The current theory is that high concentrations of greenhouse gases such as methane and carbon dioxide kept Earth from freezing. A clear divide exists in the research as to whether carbon dioxide or methane was the greenhouse gas most responsible for warming the Earth. While this article does not seek to evaluate the arguments of either greenhouse gas, the competing theories are important when discussing the evolution of life and its impact on Earth’s early atmospheres.

V.	Methane Solution to Faint Young Sun Paradox

Proponents of the methane theory suggest that because of methane’s long residence time in anoxic environments and its strong greenhouse effect, methane could be responsible for warming the planet. Methane has its absorption spectra in water vapor’s atmospheric window (Fig.1), thus an atmosphere high in methane and water vapor could produce a strong greenhouse effect. However, although methane is released from hydrothermal vents in the sea floor, it is highly soluble in water. Thus, the only way that methane could have been built up in the atmosphere is by the development of methanogenic bacteria. Rock records indicate that methane was present in large quantities during Earth’s early atmosphere. Opponents of the methane theory point to methane’s high water solubility as the main reason why methane could never have built up a high enough concentration to solely keep Earth above freezing.



VI. Carbon Dioxide Solution to Faint Young Sun Paradox

The carbon dioxide greenhouse theory answers many of the unknowns in the methane theory, but it is also fraught with issues. Like methane, carbon dioxide has its absorption spectra in water vapor’s atmospheric window (Fig.1). One of Earth’s earliest life forms, cyanobacteria, released carbon dioxide in large quantities. As carbon dioxide built up in the oceans and then the atmosphere due to cyanobacteria, it would have warmed the planet above freezing. Opponents of the theory counter that weathering on Earth’s surface would have consumed much of the carbon dioxide and, indeed, rock records indicate that, while weathering was occurring, there was 20 times too little carbon dioxide to keep Earth above freezing.

VII. Development of Life

The answer to the competing theories may lie in compromise. Because the atmosphere at this time was rich in nitrogen, hydrogen, methane, and water, sparks like lightning were capable of producing amino acids and sugars necessary to the evolution of life. Eventually, single-celled organisms like methanogens and cyanobacteria formed in the oceans and maintained or increased the levels of carbon dioxide and methane. As both methane and carbon dioxide increased in concentration in the atmosphere, the Earth would have warmed substantially past the freezing point of water . In fact, the hot temperatures created by these two gases’ greenhouse effects would have produced a thick, organic haze similar to the moon Titan. The effect of this thick haze is two-fold. First, the haze would have protected the Earth’s surface and shallow waters from harmful UV radiation, and thus allowed the evolution of life in the upper ocean rather than in the deeper oceans where methanogens and cyanobacteria thrived. Second, the haze would have shielded the Earth from some incoming solar radiation, thus cooling the Earth to temperatures conducive for oxygen photosynthesizers but hostile to methane photosynthesizers.

VIII. Earth’s Third Atmosphere

With the atmosphere now conducive to photosynthetic oxygen production, the amount of atmospheric oxygen began to increase steadily. Though some scientists believe photo-dissociation of oxygen from hydrogen may have been a factor in increasing oxygen levels, biological photosynthetic oxygen production was an exponentially larger factor. Meanwhile methane, which has a very short residence life in aerobic conditions, began to decrease in the atmosphere and oceans as oxygen began to rise, leading methane to evolve into higher hydrocarbons . As methane decreased in the atmosphere and oceans due to solubility, production of methane decreased as well because methanogenic bacteria cannot function in even minimally oxygenated environments; thus a positive feedback of methane depletion caused a rapid decline in atmospheric and oceanic methane.

IX. Timing of Oxidation of Earth’s Atmosphere

The timing of the oxidation of Earth’s oceans and atmosphere has come into much clearer focus during the last decade. The accepted range for the oxidation of Earth’s oceans based on rock records is 2.45 billion years ago to 2.22 billion years ago. This date range is largely based on red iron beds on the sea floor. As oxygen increased in the ocean, it began to oxidize the iron it contained, which consequently turned the iron red. The first red iron bed formations formed around 2.45 billion years ago, indicating the upper bound of the oxidation of the sea. Once most of the iron in the sea was oxidized, oxygen could escape to the atmosphere. Evidence for the timing of atmospheric oxidation is found in sulfate isotopes. Diagenetic pyrite in the Rooihoogte-Timeball Hill rock formation further narrows down the range of atmospheric oxidation to between 2.45 billion years ago and 2.32 billion years ago. The pyrite, which can be dated through the sulfur, formed 2.32 billion years ago and is embedded in a carbon-rich rock bed. Therefore, it can be assumed that the organic layer in which the pyrite is embedded is at least 2.32 billion years old, and thus oxidation of the atmosphere must have occurred prior to 2.32 billion years ago. While the newly oxidized atmosphere was not exactly the same as today’s atmosphere, due to the fact that land plants and animals had yet to evolve, this third atmosphere was far more similar to the present than any of Earth’s older atmospheres.

X.	Review

In the span of just over 2 billion years, Earth had undergone three completely different atmospheric compositions. Earth’s earliest atmosphere was made up of light elements found in the planetary nebula that formed the solar system. After these elements were outgassed, volcanism, as it often has in Earth’s history, changed the atmospheric chemistry of the atmosphere which allowed life to evolve. Life on the planet, however, was not an idle bystander to the changing atmosphere but rather was a catalyst for stark changes in oceanic and atmospheric composition. Single-cell bacteria kept the planet from freezing over due to their methane and carbon dioxide emissions, yet when the planet responded to these emissions by warming beyond the ideal temperature range of the bacteria, life further evolved and caused the oxidation of Earth’s ocean and atmosphere. Though the complex interactions between gases, water, and life make Earth’s early atmospheres difficult to resolve, it is this complexity that allowed formation of life as we know it today.

XI. Ongoing Research

Further research on Earth’s early climate is needed. Recently, scientists at Rensselaer Polytechnic Institute have proposed that perhaps Earth’s early atmosphere was not as oxygen-poor as previously thought. By studying the chemical makeup of magma and its interactions with Earth’s crust and atmosphere, it is apparent that a connection exists between the atmospheric oxidation levels and the presence of certain minerals, particularly the cerium content within zircon. Researchers have found that cerium exists in two different states of oxidation. An increased presence of the more oxidized form is directly correlated to increased atmospheric oxygen levels following igneous rock formation. Other studies should look at whether outside events, such as large meteor impacts, could have played a role in bringing water and gases to Earth. In addition to these studies, much of the current research focuses on dating the transition times of Earth’s early atmospheres.