작성일 : 21-01-07 15:00
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[주니어] 2021년 01월 07일 해태분 -김채운
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RC 수업
rc_final test1 3 : Earth’s Atmosphere
Earth’s Atmosphere
The Earth’s atmosphere consists mainly of heavy gases like nitrogen, but lacks light gases such as hydrogen and helium. This is odd, because nitrogen is known to be the seventh most abundant element in the universe, whereas hydrogen is the most common, which begs the question of why hydrogen is no longer abundant in Earth’s atmosphere. Earth, currently the only planet known to support life, has an atmosphere distinguishable from others in the solar system, due to being composed of byproducts of life, such as oxygen. This distinction can be seen in the changes observed through studies done by a series of scientists, which have ultimately classified Earth’s atmosphere into three stages of evolution: primordial atmosphere, secondary atmosphere, and the addition of oxygen.
Initially, light gases were abundant in Earth’s primordial atmosphere, but gradually started to decline. This can be explained through ‘Jeans escape’, a classical form of thermal escape based on Maxwell’s Distribution that prescribes that the kinetic energy distribution of molecules is dependent on the mass and velocity of the molecule. From this, one can conclude that the bigger the molecule, the lower its average velocity at a given temperature, which makes escape more unlikely. Furthermore, the greater the mass of the planet, the higher the required escape velocity; which ultimately results in less escaping molecules. Thus, giant planets with high gravity possess higher concentrations of light gases such as hydrogen and helium, which are rare in the inner planet’s atmospheres. The distance from the star also affects molecular velocity; the closer the planet, the hotter the atmosphere becomes, leading to higher molecular velocity and higher escape rates. This is how Saturn’s relatively small moon, Titan, keeps its atmosphere. Even though its gravitational pull is lower than that of Earth, its cooler temperature allows it to hold on to its atmosphere.
Scientists believe that as light gases escaped from Earth’s atmosphere, nitrogen began constituting a larger part of it after volcanic eruptions spewed gases, ultimately forming the main component of the secondary atmosphere. Three primary reasons why only nitrogen filled the environment are that it is volatile even at low temperatures, unreactive with the materials that make up the earth’s crust, and very stable in the presence of solar radiation. In contrast to more abundant elements which are major components of Earth’s crust, nitrogen is not stable as a part of a crystal lattice, and will not bond with the others to form the terrestrial earth; it, instead, remains a gas. Unlike nitrogen, other elements are unstable and easily react with each other in the atmosphere as well as break down when exposed to solar radiation. Thus, over time, nitrogen built up in the atmosphere to a much greater extent than other elements.
One theory introducing the presence of atmospheric oxygen is the Gaia theory. It is believed by scientists that the primordial atmosphere of Earth did not contain oxygen and that the appearance of this gas required the evolution of photosynthetic life forms, such as early cyanobacteria and later single-celled algae, which emerged as the earth cooled and the gaseous water condensed and formed oceans. As a result of photosynthesis, molecular oxygen built up to the present level, which appears to have been relatively stable for several billion years. If this level were even slightly higher, then the earth’s biomass would be combustible, leading to extensive explosions and forest fires, severely damaging earth’s ecosystem. The Gaia theory, thus, suggests that there is a planetary, homeostatic control over the oxygen concentration in the atmosphere that balances the ratio of organisms producing oxygen to those consuming it.
However, some argue that the amount of oxygen collected in the Earth’s atmosphere cannot be accounted for entirely by cyanobacteria. They believe that the theory is not adequate because it can only account for 1 percent to 10 percent concentration levels, while today’s atmosphere contains 21 percent. Thus, a second theory called the planetary atmosphere tectonic machine theory was developed. The theory posits that all of earth’s oxygen came from not life forms, but the ocean, near the tectonic plates where magma is released and water comes into contact with temperatures over 2000°C. The super-heated water is then converted into hydrogen and oxygen. The oxygen bubbles then oxygenate the ocean, and are released into the atmosphere when they reach the surface. This oxygen in the atmosphere is then shielded from being fully lost to space by the ozone and the earth’s magnetic field.
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