Friday, June 5, 2009

The evolution of the earth’s early atmosphere.

How did Earth's early atmosphere evolved.
by
Dr. Nitish Priyadarshi

The Earth's atmosphere (or air) is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. It has a mass of about five quadrillion metric tons. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, and trace amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night.

The solid earth accumulated about 4700 m.y. ago from a cloud of cosmic particles and gaseous materials and as they collected gravitationally a hot planetary nucleus formed. This nucleus eventually became the present core as the mantle and crust consolidated. An atmosphere probably existed even in these early stages of the first billion years of earth’s history, though it was apparently transitory. Judging from the atmospheres of the major planets, Jupiter and Saturn, which retain light elements by virtue of their large gravitational attraction, hydrogen and helium would have been abundant in earth’s primordial atmosphere. These elements were derived in part from the original gaseous material of the cosmic cloud, but volcanic outgassing during lithification of the crust probably continued as well. Neon and argon and some of the lighter gases such as xenon probably also existed in the early atmosphere.

The first atmosphere of the earth, then, contained hydrogen, helium, neon, argon and various other lighter and inert gases, none of which is abundant in the present atmosphere. Most of these on liberation to the air now either escape earth’s gravitational pull because of their low densities or are bound up in minerals by chemically reacting with them. It is likely that the primitive atmosphere did not linger long but was dissipated through these processes.

A little reflection tells us that earth’s present atmosphere necessarily evolved from one that was different. We know no primary source for the free molecular oxygen that comprises one –fifth of our present atmosphere. Compared with solar abundances, our atmosphere has only traces of hydrogen and helium but a disproportionate amount of nitrogen.

An important clue to the origin of our ancestral atmosphere is found in the abundances of so-called noble gases – elements that, unlike oxygen, do not (or rarely) combine with others because they have the stable configuration of 8 (or 2 in the case of helium) in their outermost shell of electrons. As they do not ordinarily lose, gain, or share electrons with other elements, variations in their abundance imply different sources. Had earth inherited its atmosphere directly from the solar nebula, the gaseous elements neon, argon, krypton, xenon, and radon should be present in approximately solar abundances, allowing for the addition of radiogenic isotopes. That is not the case. It has been repeatedly noted over the past half-century that all the noble gases are grossly depleted in the earth’s atmosphere compared with solar and cosmic abundances. They are depleted, in fact, by several to many orders of magnitude. This means either that earth accumulated without an atmosphere of nebular proportions or that any initial atmosphere escaped its gravity field in some subsequent episode of heating that accelerated even the heavy noble gases to escape velocities.

The most significant development following sufficient cooling and consolidation of the surface rocks was liberation of abundant water along with CO2 , N2, and H2 S by volcanic outgassing. Water vapor is dissociated in the upper atmosphere by ultraviolet light to yield oxygen and hydrogen. This process constituted the sole source of free oxygen of the early atmosphere, and the build up to significant oxygen concentrations occupied the long interval between at least 3400 and about 2000 m.y. ago. Further, oxygen of the early high atmosphere was photochemically converted to ozone as at present, and with time, ozone concentration led to the development of a screen to ultraviolet light. Lastly, accumulation of water molecules in the atmosphere caused extensive precipitation and hence the initiation of the oceans at some time prior to 3760 m.y. ago, when the oldest known sedimentary rocks were deposited.

Other concept regarding evolution of early oxygen in atmosphere:
If earth’s primitive atmosphere resulted from volcanic outgassing, we have a problem, because volcanoes do not emit free oxygen. Where did the very significant percentage of oxygen in our present atmosphere (20 percent) come from?

The major source of oxygen is green plants. Plants did not just adapt to their environment, they actually influenced it, dramatically altering the composition of the entire planet’s atmosphere by using carbon dioxide and releasing oxygen. This is a good example of how earth operates as a giant system in which living things interact with their environment.

How did plants come to alter the atmosphere? The key is the way in which plants create their own food. They employ photosynthesis, in which they use light energy to synthesize food sugars from carbon dioxide and water. The process releases a waste gas, oxygen. Those of us in the animal kingdom rely on oxygen to metabolize our food, and we in turn exhale carbon dioxide as a waste gas. The plant use this carbon dioxide for more photosynthesis, and so on, in a continuing system.

The first life-forms on earth, probably bacteria, did not need oxygen. Their life processes were geared to the earlier, oxygen less atmosphere. Even today, many anaerobic thrive in environments that lack free oxygen. Later, primitive plants evolved that used photosynthesis and released oxygen. Slowly, the oxygen content of earth’s atmosphere increased. The Precambrian rock record suggests that much of the first free oxygen did not remain free because it combined with (oxidized) other substances dissolved in water, especially iron. Iron has tremendous affinity for oxygen, and the two elements combine to form iron oxides (rust) at any opportunity. To this day, the majority of oxygen produced over time is locked up in the ancient "banded rock" and "red bed" formations.

Then, once the available iron satisfied its need for oxygen, substantial quantities of oxygen accumulated in the atmosphere. By the beginning of the Paleozoic era, about 4 billion years into earth’s existence, the fossil record reveals abundant ocean- dwelling organisms that require oxygen to live.
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.
Hence, the composition of earth’s atmosphere has evolved together with its life-forms, from an oxygen less envelop to today’s oxygen-rich environment.

Sources:
Cloud,P. 1988. Oasis in space, earth history from the beginning. W.W. Norton & Company, New York.
Frakes, L. A. 1979. Climates throughout geologic times. Elsevier, New York.
Tarbuck, E.J. and Lutgens, F.K. 1994. Earth Science. Prentice Hall, New Jersey.
http://knowledgerush.com/kr/encyclopedia/Earth's_atmosphere/
http://en.wikipedia.org/wiki/Earth's_atmosphere

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