Atmospheric evolution

Considerable uncertainty exists about the origin and composition of the Earth's earliest atmosphere. Many models assume that methane and ammonia dominated the planet's early atmosphere, instead of nitrogen and carbon dioxide, as it is presently. The gases that formed the early atmosphere could have come from outgassing by volcanoes, from extraterrestrial sources (principally cometary impacts), or, most likely, both. Alternatively, comets may have brought organic molecules to Earth. A very large late impact is thought to have melted outer parts of the Earth, formed the Moon, and blown away the earliest atmosphere. The present atmosphere must therefore represent a later, secondary atmosphere formed after this late impact.

The earliest atmosphere and oceans of the Earth probably formed from early degassing of the interior by volcanism within the first 50 million years of Earth history. It is likely that our present atmosphere is secondary, in that the first, or primary, atmosphere would have been vaporized by the late great impact that formed the Moon, if it survived being blown away by an intense solar wind when the sun was in a T-Tauri stage of evolution. The primary atmosphere would have been composed of gases left over from accretion, including primarily hydrogen, helium, methane, and ammonia, along with nitrogen, argon, and neon. since the atmosphere has much less than the expected amount of these elements, however, and is quite depleted in these volatile elements relative to the sun, it is thought the primary atmosphere has been lost to space.

Gases are presently escaping from the Earth during volcanic eruptions, and also being released by weathering of surface rocks. The secondary atmosphere was most likely produced from degassing of the mantle by volcanic eruptions, and perhaps also by cometary impact. Gases released from volcanic eruptions include N, S, C02, and H20, closely matching the suite of volatiles that the present atmosphere and oceans comprise. But there was little or no free oxygen in the early atmosphere, as oxygen was not produced until later, by photosynthetic life.

The early atmosphere was dense, with H2o, C02, S, N, HCl. The mixture of gases in the early atmosphere would have made greenhouse conditions similar to that presently existing on Venus. But, since the early Sun during the Hadean Era was approximately 25 percent less luminous than today, the atmospheric greenhouse kept temperatures close to their present range, where water is stable

Thermosphere Mesosphere Stratosphere Troposphere

The doldrums

Equatorial lows

Horse latitudes

Polar front Polar high

Thermosphere Mesosphere Stratosphere Troposphere

The doldrums

Equatorial lows

Horse latitudes

Polar cell „-

Stratosphere

Polar front jet

Subtropical jet

Polar front jet

Subtropical jet

Stratosphere

Polar cell

Polar cell

Hadley cell

" il Ferrel cell

Hadley cell

" il Ferrel cell

pole

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equator

Major atmosphere circulation patterns on the Earth, in map view (top), and in cross section (bottom)

and life can form and exist. As the Earth cooled, water vapor condensed to make rain that chemically weathered igneous crust, making sediments. Gases dissolved in the rain made acids, including carbonic acid (H2Co3), nitric acid (HNo3), sulfuric acid (H2so4), and hydrochloric acid (ho). These acids were neutralized by minerals (which are bases) that became sediments, and chemical cycling began. These waters plus dissolved components became the early hydrosphere, and chemical reactions gradually began changing the composition of the atmosphere, getting close to the dawn of life.

aurora, aurora borealis, aurora australis

During the early Archean, the Sun was only about 70 percent as luminous as it is presently, so the Earth must have experienced a greenhouse warming effect to keep temperatures above the freezing point of water, but below the boiling point. Increased levels of carbon dioxide and ammonia in the early atmosphere could have acted as greenhouse gases, accounting for the remarkable maintenance of global temperatures within the stability field of liquid water, allowing the development of life. Much of the carbon dioxide that was in the early atmosphere is now locked up in deposits of sedimentary limestone, and in the planet's biomass. The carbon dioxide that shielded the early Earth and kept temperatures in the range suitable for life to evolve now forms the bodies and remains of those very life-forms.

See also aurora, Aurora Borealis, Aurora Australis; climate; climate change; greenhouse effect; weathering.

(more than 1 million km/hr) as a plasma known as the solar wind. When these charged particles move close to Earth, they interact with the magnetic field, changing its shape in the process. The natural undisturbed state of the Earth's magnetic field is broadly similar to a bar magnet, with magnetic flux lines (of equal magnetic intensity and direction) coming out of the south polar region, and returning into the north magnetic pole. The solar wind deforms or distorts this ideal state into an elongate teardrop-shaped configuration known as the magnetosphere. The magnetosphere has a rounded compressed side facing the Sun, and a long tail (magnetotail) on the opposite side that stretches past the orbit of the moon. The magnetosphere shields the Earth from many of the charged particles from the Sun by deflecting them around the edge of the magnetosphere, causing them to flow harmlessly into the outer solar system.

FURTHER READING

Ahrens, C. Donald. Meteorology Today. 7th ed. Pacific Grove, Calif.: Brooks/Cole, 2002.

Ashworth, William, and Charles E. Little. Encyclopedia of Environmental Studies, New Edition. New York: Facts On File, 2001.

Bekker, Andrey, H. Dick Holland, P. L. Wang, D. Rumble III, H. J. Stein, J. L. Hannah, L. L. Coetzee, and Nick Beukes, "Dating the Rise of Atmospheric Oxygen." Nature 427 (2004): 117-120.

Kasting, James F. "Earth's Early Atmosphere." Science 259 (1993): 920-925.

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