by Isaac Asimov
Finally, there are in orbit numerous spy satellites designed to be able to detect military movements, military concentrations and stores, and so on. There are not lacking people who plan to make space another arena for war or to develop killer satellites designed to strike down enemy satellites, or to place advanced weapons in space which can strike more quickly than Earth-based weapons. This is the demonic side of space exploration, even though it only marginally increases the speed with which a full-scale thermonuclear war can destroy civilization.
The stated purpose of “keeping the peace” by discouraging the other side from making war is proclaimed by both superpowers, the United States and the Soviet Union. The acronym for this theory of peace by “mutual assured destruction,” with each side knowing that starting a war will bring about its own destruction as well as that of the other side, is MAD—and mad it is, for increasing the quantity and the deadliness of armaments has never hitherto prevented war.
The Gases in Air
THE LOWER ATMOSPHERE
Up to modern times, air was considered a simple, homogeneous substance. In the early seventeenth century, the Flemish chemist Jan Baptista van Helmont began to suspect that there were chemically different gases. He studied the vapor given off by fermenting fruit juice (carbon dioxide) and recognized it as a new substance. Van Helmont was, in fact, the first to use the term gas—a word he is supposed to have coined, about 1620, from chaos, the Greek word for the original substance out of which the universe was made. In 1756, the Scottish chemist Joseph Black studied carbon dioxide thoroughly and definitely established it as a gas other than air. He even showed that small quantities of it existed in the air. Ten years later, Henry Cavendish studied a flammable gas not found in the atmosphere. It was eventually named hydrogen. The multiplicity of gases was thus clearly demonstrated.
The first to realize that air was a mixture of gases was the French chemist Antoine-Laurent Lavoisier. In experiments conducted in the 1770s, he heated mercury in a closed vessel and found that the mercury combined with part of the air, forming a red powder (mercuric oxide), but four-fifths of the air remained a gas. No amount of heating would consume any of this remaining gas. A candle would not burn in it, nor could mice live in it.
Lavoisier decided that air was made up of two gases. The one-fifth that combined with mercury in his experiment was the portion of the air that supports life and combustion: this he called oxygen. The remainder he called azote, from Greek words meaning “no life.” Later it became known as nitrogen, because the substance was present in sodium nitrate, commonly called niter. Both gases had been discovered in the previous decade. Nitrogen had been discovered in 1772 by the Scottish physician Daniel Rutherford; and oxygen, in 1774 by the English Unitarian minister Joseph Priestley.
This alone is sufficient to demonstrate that Earth’s atmosphere is unique in the solar system. Aside from Earth, seven worlds in the solar system arc known to have an appreciable atmosphere. Jupiter, Saturn, Uranus, and Neptune (the first two, certainly; the latter two, probably) have hydrogen atmospheres, with helium as a minor constituent. Mars and Venus have carbon dioxide atmospheres, with nitrogen as a minor constituent. Titan has a nitrogen atmosphere with methane as a minor constituent. Earth alone has an atmosphere nearly evenly split between two gases, and Earth alone has oxygen as a major constituent. Oxygen is an active gas and, from ordinary chemical considerations, it would be expected that it would combine with other elements and would disappear from the atmosphere in its free form. This is something we will return to later in the chapter; but for now, let us continue dealing with the further details of the chemical composition of air.
By the mid-nineteenth century, the French chemist Henri Victor Regnault had analyzed air samples from all over the world and discovered the composition of the air to be the same everywhere. The oxygen content was 20.9 percent, and it was assumed that all the rest (except for a trace of carbon dioxide) was nitrogen.
Nitrogen is a comparatively inert gas; that is, it does not readily combinr with other substances. It can, however, be forced into combination=for instance, heating it with magnesium metal forms the solid magnesium III tride. Some years after Lavoisier’s discovery, Henry Cavendish tried to exhaust the nitrogen by combining it with oxygen under the influence of an electric spark. He failed. No matter what he did, he could not get rid of a small bubble of remaining gas, amounting to less than 1 percent of the original quantity. Cavendish thought this might be an unknown gas, even more inert than nitrogen. But not all chemists are Cavendishes, and the puzzle was not followed up, so the nature of this residue of air was not discovered for another century.
In 1882, the British physicist Robert John Strutt, Lord Rayleigh, compared the density of nitrogen obtained from air with the density of nitrogen obtained from certain chemicals and found, to his surprise, that the air nitrogen was definitely denser. Could it be that nitrogen obtained from air was not pure but contained small quantities of another, heavier gas? A Scottish chemist, Sir William Ramsay, helped Lord Rayleigh look further into the matter. By this time, they had the aid of spectroscopy. When they heated the small residue of gas left after exhaustion of nitrogen from air and examined its spectrum, they found a new set of bright lines—lines that belonged to no known element. To their newly discovered, very inert element they gave the name argon (from a Greek word meaning “inert”).
Argon accounted for nearly all of the approximately 1 percent of unknown gas in air—but there were still several trace constituents in the atmosphere, each constituting only a few parts per million. During the 1890s Ramsay went on to discover four more inert gases: neon (“new”), krypton (“hidden”), xenon (“stranger”), and helium, which had been discovered more than thirty years before in the sun. In recent decades, the infrared spectroscope has turned up three others: nitrous oxide (“laughing gas”), whose origin is unknown; methane, a product of the decay of organic matter; and carbon monoxide. Methane is released by bogs, and some 45 million tons of the same gas, it has been calculated, are added to the atmosphere each year by the venting of intestinal gases by cattle and other large animals. The carbon monoxide is probably man-made, resulting from the incomplete combustion of wood, coal, gasoline, and so on.
THE STRATOSPHERE
I have so far been discussing the composition of the lowest reaches of the atmosphere. What about the stratosphere? Teisserenc de Bort believed that helium and hydrogen might exist in some quantity up there, floating on the heavier gases underneath. He was mistaken. In the middle 1930s, Russian balloonists brought down samples of air from the upper stratosphere, and it proved to be made up of oxygen and nitrogen in the same l-to-4 mixture as the air of the troposphere.
But there were reasons to believe some unusual gases existed still higher in the upper atmosphere, and one of the reasons was the phenomenon called the airglow. This is the very feeble general illumination of all parts of the night sky, even in the absence of the moon. The total light of the airglow is considerably greater than that of the stars, but is so diffuse that it is not noticeable except to the delicate light-gathering instruments of the astronomer.
The source of the light had been a mystery for many years. In 1928, the astronomer V. M. Slipher succeeded in detecting in the airglow some mysterious spectral lines that had been found in nebulae in 1864 by William Huggins and were thought to represent an unfamiliar element, named nebulium. In 1927, through experiments in the laboratory, the American astronomer Ira Sprague Bowen showed that the lines came from atomic oxygen: that is, oxygen existing as single atoms and not combined in the normal form of the two-atom molecule. Similarly, other strange spectral lines from the aurora turned out to represent atomic nitrogen. Both atomic oxygen and atomic nitrogen in the upper atmosphere are produced by energetic radiation from the sun, which breaks down the molecules into single atoms—a possibility first suggested in 1931 by Sydney Chapman. Fortunately the high-energy radiation is, in this way, absorbed or weakened before it reaches the lowe
r atmosphere.
The airglow, Chapman maintained, comes from the recombination at night of the atoms that are split apart by solar energy during the day. In recombining, the atoms give up some of the energy they absorbed in splitting, so that the airglow is a kind of delayed and very feeble return of sunlight in a new and specialized form. Experiments in 1956—both in the laboratory and, through rockets, in the upper atmosphere, under the direction of Murray Zelikoff—supplied direct evidence of this theory. Spectroscopes carried by the rockets recorded the green lines of atomic oxygen most strongly at a height of 60 miles. A smaller proportion of the nitrogen was in the atomic form, because nitrogen molecules hold together more strongly than do oxygen molecules; nevertheless, the red light of atomic nitrogen was strong at a height of 95 miles.
Slipher had also found lines in the airglow that were suspiciously like well-known lines emitted by sodium. The presence of sodium seemed so unlikely that the matter was dropped in embarrassment. What would sodium, of all things, be doing in the upper atmosphere? It is not a gas, after all, bill a very reactive metal that does not occur alone anywhere on the earth. It is always combined with other elements, most commonly in sodium chloride (table salt). But, in 1938, French scientists established that the lines were indeed identical with the sodium lines. Unlikely or not, sodium had to be in the upper atmosphere. Again, rocket experiments clinched the matter: then spectroscopes recorded the yellow light of sodium unmistakably, and most strongly at a height of 55 miles. Where the sodium comes from is still a mystery—perhaps from ocean salt spray or from vaporized meteors. Still more puzzling is the fact that lithium—a rare relative of sodium—was also found. in 1958, to be contributing to the airglow.
In the course of their experiments, Zelikoff’s team produced an artificial airglow. They fired a rocket that at miles released a cloud of nitric oxide gas This accelerated the recombination of oxygen atoms in the upper atmosphere. Observers on the ground easily sighted the bright glow that resulted. A similar experiment with sodium vapor also was successful: it created a clearly visible, yellow glow. When Soviet scientists sent Lunik III in the direction of the moon in October 1959, they arranged for it to expel a cloud of sodium vapor as a visible signal that it had gone into orbit.
At lower levels in the atmosphere, atomic oxygen disappears, but the solar radiation is still energetic enough to bring about the formation of the three-atom variety of oxygen called ozone. The ozone concentration is greatest at a height of 15 miles. Even there, in what is called the ozonosphere (first discovered in 1913 by the French physicist Charles Fabry), it makes up only 1 part in 4 million of the air, but that is enough to absorb ultraviolet light sufficiently to protect life on the earth.
Ozone is formed by the combination of atomic oxygen (a single atom) with ordinary oxygen molecules (two atoms). Ozone does not accumulate to Iarge amounts, for it is unstable. The three-atom molecule easily breaks down to the much more stable two-atom form by the action of sunlight, by the nitrous oxide that occurs naturally in tiny amounts in the atmosphere, and by other chemicals. The balance between formation and breakdown leaves, in the ozonosphere at all times, the small concentration referred to; and its shield against the sun’s ultraviolet (which would break down many of the delicate molecules essential to living tissue) has protected life since oxygen first entered Earth’s atmosphere in quantity.
The ozonosphere is not far above the tropopause and varies in height in the same way, being lowest at the poles and highest at the Equator. The ozonosphere is richest in Ozone at the poles and poorest at the Equator where the breakdown effect of sunlight is highest.
It would be dangerous if human technology were to produce anything that would accelerate ozone breakdown in the upper atmosphere and weaken the ozonosphere shield. The weakening of the shield would increase the ultraviolet incidence at Earth’s surface, which would, in turn, increase the incidence of skin cancer—especially among fair-skinned people. Some have estimated that a 5-percent reduction in the ozone shield could result in 500,000 additional cases of skin cancer each year over the world in general. Ultraviolet light, if increased in concentration, might also affect the microscopic life (plankton) in the sea surface with possible fearful consequences, since plankton forms the base of the food chain in the sea and, to a certain extent, on land as well.
There is indeed some danger that human technology will affect the ozonosphere. Increasingly, jet planes are Hying through the stratosphere, and rockets are making their way through the entire atmosphere and into space. The chemicals poured into the upper atmosphere by the exhausts of these vehicles might conceivably accelerate ozone breakdown. The possibility was used as an argument against the development of supersonic planes in the early 1970s.
In 1974, spray cans were unexpectedly found to be a possible danger. These cans use imprisoned Freon (a gas that will be mentioned again, in chapter 11) as a source of pressure that serves to drive out the contents of the can (hairspray, deodorants, air-fresheners, or whatever) in a fine spray. Freon itself is, chemically, as harmless as one can imagine a gas to be——colorless, odorless, inert, and unreactive, without any effect on human beings. About 1,700,000,000 pounds of it were being released into the atmosphere from spray cans and other devices each year at the time its possible danger was pointed out.
The gas, reacting with nothing, spreads slowly through the atmosphere and finally reaches the ozonosphere where it might serve to accelerate the breakdown of ozone. This possibility was raised on the basis of laboratory tests. Whether it would actually do this under the conditions of the upper atmosphere is somewhat uncertain, but the possibility represents too great a danger to dismiss in cavalier fashion. The use of spray cans with Freon has vastly decreased since the controversy began.
However, Freon is used to a much greater extent in air-conditioning and in refrigeration, where it has not been easily given up or even replaced. Thus, the ozonosphere remains at hazard, for, once formed, Freon is bound sooner or later to be discharged into the atmosphere.
THE IONOSPHERE
Ozone is not the only atmospheric constituent that is far more prominent at great heights than in the neighborhood of the surface. Further rocket experiments showed that Teisserenc de Bort’s speculations concerning layers of helium and hydrogen were not wrong but merely misplaced. From 200 to 600 miles upward, where the atmosphere has thinned out to near-vacuum, there is a layer of helium, now called the heliosphere. The existence of this layer was first deduced in 1961 by the Belgian physicist Marcel Nicolet from the frictional drag on the Echo I satellite. This deduction was confirmed hy actual analysis of the thin-gas surroundings by Explorer XVII, launched on 2 April 1963.
Above the heliosphere is an even thinner layer of hydrogen, the protonosphere, which may extend upward some 40,000 miles before quite fading off into the general density of interplanetary space.
High temperatures and energetic radiation can do more than force atoms apart or into new combinations. They can chip electrons away from atoms and so ionize the atoms. What remains of the atom is called an ion and differs from an ordinary atom in carrying an electric charge. The word ion, first coined in the 1830s by the English scholar William Whewell, comes from a Greek word meaning “traveler.” Its origin lies in the fact that when an electric current passes through a solution containing ions, the positively charged ions travel in one direction, and the negatively charged ions in the other.
A young Swedish student of chemistry named Svante August Arrhenius was the first to suggest, in 1884, that ions are charged atoms, as the only means of explaining the behavior of certain solutions that conducted an electric current. His notions, advanced in the thesis he presented for his degree of doctor of philosophy in that year, were so revolutionary that his examiners could scarcely bring themselves to pass him. The charged particles within the atom had not yet been discovered, and the concept of an electrically charged atom seemed ridiculous. Arrhenius got his degree, but with only a minimum passing grade.
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When the electron was discovered in the late 1890s (see chapter 6), Arrhenius’s theory suddenly made startling sense. He was awarded the Nobel Prize in chemistry in 1903 for the same thesis that nineteen years earlier had nearly lost him his doctoral degree. (This sounds like an improbable movie scenario, I admit, but the history of science contains many episodes that make Hollywood seem unimaginative.)
The discovery of ions in the atmosphere did not emerge until after Guglielmo Marconi started his experiments with wireless. When, on 12 December 1901, he sent signals from Cornwall to Newfoundland, across 2,100 miles of the Atlantic Ocean, scientists were startled. Radio waves travel only in a straight line. How had they managed to go around the curvature of the earth and get to Newfoundland?
A British physicist, Oliver Heaviside, and an American electrical engineer, Arthur Edwin Kennelly, soon suggested that the radio signals might have been reflected back from the sky by a layer of charged particles high in the atmosphere. The Kennelly-Heaviside layer, as it has been called ever since, was finally located in 1922. The British physicist Edward Victor Appleton discovered it by paying attention to a curious fading phenomenon in radio transmission. He decided that the fading was the result of interference between two versions of the same signal: one coming directly from the transmitter to his receiver; the other, by a roundabout route via reflection from the upper atmosphere. The delayed wave was out of phase with the first, so the two waves canceled each other; hence, the fading.
It was a simple matter then to find the height of the reflecting layer. All Appleton had to do was send signals at such a wavelength that the direct signal completely canceled the reflected one: that is, the two signals arrived at opposite phases. From the wavelength of the signal used and the velocity of radio waves, he could calculate the difference in the distances the two trains of waves had traveled. In this way, he determined, in 1924, that the Kennelly-Heaviside layer was some 65 miles up.