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EARTH SCIENCE

ON THE DISCOVERY OF THE STRATOSPHERE

The atmosphere of Earth is divisible into several layers, each layer having a characteristic temperature range, pressure range, and composition. The layers, from the surface of Earth, are (with thicknesses varying at different latitudes): troposphere (0 to approximately 10 kilometers), stratosphere (from approximately 10 to 50 kilometers), mesosphere (approximately 50 to 80 kilometers), thermosphere (approximately 80 to 500 kilometers), and exosphere (above approximately 500 kilometers. Other layers, essentially meta-layers, are also recognized: a) the "chemosphere" is the region between approximately 32 and 92 kilometers where many important chemical reactions occur; b) the "ionosphere", above approximately 80 kilometers, is a shell of high electron concentration resulting from very short wavelength sunlight stripping electrons from atoms and molecules (mainly oxygen and nitrogen) to create an ionized layer; c) the magnetosphere is the constantly changing magnetic field generated by the Earth's dynamo, this magnetic field influencing the behavior of electrically charged particles, and the field extending approximately 10 Earth radii (64,000) kilometers into space on the sunward side.

The boundary between troposphere and stratosphere is called the "tropopause"; that between stratosphere and mesosphere is called the "stratopause"; and that between mesosphere and thermosphere is called the "mesopause", in each case the root "pause" used because of an inflection in the temperature-altitude curve.

The temperature of the atmosphere undergoes marked but systematic variation with altitude. In the troposphere, the layer closest to the surface, the temperature decreases by approximately 6.5 degrees centigrade per kilometer of altitude, until at the tropopause (10 to 11 kilometers) the temperature stabilizes at approximately -53 degrees centigrade. The temperature remains stable in the stratosphere, and even increases with altitude to approximately 0 degrees centigrade at the stratopause. Then in the mesosphere there occurs again a decline in temperature with altitude, now down to -100 degrees centigrade, and then after the mesopause and into the upper atmosphere (thermosphere and exosphere), the temperature rises markedly in these regions of extremely low air density, so that at 200 kilometers the temperature range is 300 to 900 degrees centigrade, depending on solar radiance.

The first hint that Earth's atmosphere is a series of concentric shells was provided by the meteorologist Leon Teisserenc de Bort (1855-1913) [the surname is Teisserenc de Bort]. From 1892 to 1896, Teisserenc de Bort served as chief meteorologist at the Central Meteorological Bureau in Paris, but in 1896 he resigned and carried out his meteorological balloon investigations himself at his estate near Versailles. He conducted experiments with high-flying instrumented balloons, and he was one of the pioneers in the use of such devices. He discovered that above approximately 11 kilometers the temperature, which drops steadily from sea-level to that altitude, remained constant up to the highest points he could reach. Surprised by this result, he accumulated data from 236 balloon ascents before he suggested, in 1902, that the atmosphere was divided into 2 layers. During the next few years, he termed the lower layer, the layer involving air movements, the "troposphere" ("sphere of change"), and the layer above that, a layer he mistakenly thought consisted of internal further layers, the "stratosphere" ("sphere of layers"). Thus, to Teisserenc de Bort we owe both the discovery and the name of the stratosphere.

The following points are made by Mott T. Greene (Nature 2000 407:947):

1) The author suggests that ripples from Teisserenc de Bort's discovery of the stratosphere spread far beyond meteorology. Between 1902 and 1904, the oceanographer Vagn Ekman (1874-1954) discovered similar layering of the ocean, and in 1909, the meteorologist Andrija Mohorovicic (1857-1936) used seismology to establish the existence of a similar discontinuity in the solid Earth, the discontinuity now known as the "Moho".

2) The author (Greene) also suggests that the discovery that the Earth-ocean-atmosphere system is composed of concentric shells of different density, from the core of the Earth to the top of the atmosphere, is the founding insight of modern geophysics, and that the discovery also profoundly influenced the thinking of the young meteorologist Alfred Wegener (1880-1930), leading Wegener in 1912 to propose the theory of continental drift, with the continents representing the remains of a formerly continuous Earth shell above the ocean floors. Greene concludes: "Just as air masses and ocean water masses moved under the influence of the Earth's rotation, sliding along surfaces of discontinuity, so, he [Wegener] reasoned, did the continents on a longer time-scale -- making Teisserenc de Bort not only the discoverer of the stratosphere but an honorary grandfather of continental drift."

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THE LAYERED EARTH

During the nineteenth century, the nature of the Earth's interior was a matter of fierce and fascinating debate. All theories were hampered by a lack of evidence -- the nature of rocks deep below the surface was unknown. In 1906, Richard D. Oldham observed that compressional seismic waves (P waves) slow abruptly deep within the Earth and can penetrate no further. This was strong evidence in favor of a liquid core. Three years later, Andrija Mohorovicic noticed that the velocity of seismic waves leaps from 7.2 to 8.0 km/s at around 60 km deep. He had discovered the 'Moho' seismic discontinuity that marks the crust-mantle boundary. In 1926, Beno Gutenberg obtained evidence for a seismic discontinuity at the core-mantle boundary. This, the Gutenberg discontinuity, was confirmed during the 1950s when world-wide records of blasts from underground nuclear detonations were scrutinized. Subsequent studies of the Earth's seismic properties, using seismic waves propagated by earthquakes and by controlled explosions to "x-ray" the planet (a technique called seismic tomography), have revealed a series of somewhat distinct layers or concentric shells in the solid Earth. Each shell has different chemical and physical properties.

Adapted from: Richard John Huggett: Environmental Change. Routledge, London 1997, p.56. More information at: http://www.amazon.com/exec/obidos/ASIN/041514521X/scienceweek

[The author is Senior Lecturer in Geography at the University of Manchester, UK] (Science-Week 13 Sep 99)

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CONTINENTAL DRIFT AND PLATE TECTONICS

Since the 16th century, cartographers have noticed the jigsaw-puzzle fit of the continental edges. Since the 19th century, geologists have known that some fossil plants and animals are extraordinarily similar across the globe, and some sequences of rock formations in distant continents are also strikingly alike.

At the turn of the 20th century, Austrian geologist Eduard Suess [1831-1914] proposed the theory of Gondwanaland to account for these similarities: that a giant supercontinent had once covered much or all of the Earth's surface before breaking apart to form continents and ocean basins.

A few years later, German meteorologist Alfred Wegener [1880-1930] suggested an alternative explanation: "continental drift". The paleontological patterns and jigsaw-puzzle fit could be explained if the continents had migrated across the Earth's surface, sometimes joining together, sometimes breaking apart. Wegener argued that for several hundred million years during the late Paleozoic and Mesozoic eras (200 million to 300 million years ago), the continents were united into a supercontinent that he labeled /Pangea/ -- all Earth. Continental drift would also explain paleoclimate change, as continents drifted through different climate zones and ocean circulation was altered by the changing distribution of land and sea, while the interactions of rifting and drifting land masses provided a mechanism for the origins of mountains, volcanoes, and earthquakes.

Continental drift was not accepted when first proposed, but in the 1960s it became a cornerstone of the new global theory of plate tectonics. The motion of land masses is now explained as a consequence of moving "plates" -- large fragments of the Earth's surface layer in which the continents are embedded.

Adapted from: Naomi Oreskes (Ed.): Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Westview Press 2001, p.3. Nore information: http://www.amazon.com/exec/obidos/ASIN/0813339812/scienceweek

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