Asimov's New Guide to Science

by Isaac Asimov

  The western and eastern sides of the Atlantic Ocean are a study in contrasts. Labrador, on the western side, exposed to the Labrador current, is a desolation with a total population of 25,000. On the eastern side, in precisely the same latitudes, are the British Isles with a population of 55,000,­000, thanks to the Gulf Stream.

  A current moving directly along the Equator is not subjected to the Coriolis effect and may move in a straight line. Such a thin, straight current was located in the Pacific Ocean, moving due east for several thousand miles along the Equator. It is called the Cromwell current after its discoverer, the American oceanographer Townsend Cromwell. A similar current, somewhat slower, was discovered in the Atlantic in 1961 by the American oceanographer Arthur D. Voorhis.

  Nor is circulation confined to surface currents only. That the deeps cannot maintain a dead calm is clear from several indirect forms of evidence. For one thing, the life at the top of the sea is continually consuming its mineral nutrients—phosphate and nitrate—and carrying this material down to the depths with itself after death; if there were no circulation to bring it up again, the surface would become depleted of these minerals. For another thing, the oxygen supplied to the oceans by absorption from the air would not percolate down to the depths at a sufficient rate to support life there if there were no conveying circulation. Actually oxygen is found in adequate concentration down to the very floor of the abyss. This can be explained only by supposing that there are regions in the ocean where oxygen-rich surface waters sink.

  The engine that drives this vertical circulation is temperature difference. The ocean’s surface water is cooled in polar regions and therefore sinks. This continual flow of sinking water spreads out all along the ocean floor, so that even in the tropics the bottom water is very cold—near the freezing point. Eventually the cold water of the depths wells up toward the surface, for it has no other place to go. After rising to the surface, the water warms and drifts off toward the Arctic or the Antarctic, there to sink again. The resulting circulation, it is estimated, would bring about complete mixing of the Atlantic Ocean, if something new were added to part of it, in about 1,000 years. The larger Pacific Ocean would undergo complete mixing in perhaps 2,000 years.

  The Antarctic is much more efficient in supplying cold water than the Arctic is. Antarctica has an icecap ten times as large as all the ice in the Arctic, including the Greenland icecap. The water surrounding Antarctica, made frigid by melting ice, spreads northward on the surface till it meets the warm waters carried southward from the tropical regions. The cold water from Antarctica, denser than the warm tropical waters, sinks below it at the line of the Antarctic convergence, which in some places extends as far north as 40° S.

  The cold Antarctic water spreads through all the ocean bottoms carrying with it oxygen (for oxygen, like all gases, dissolves more easily and in greater quantities in cold water than in warm) and nutrients. Antarctica (the “icebox of the world”) thus fertilizes the oceans and controls the weather of the planet.

  The continental barriers complicate this general picture. To follow the actual circulation, oceanographers have resorted to oxygen as a tracer. As the polar water, rich in oxygen, sinks and spreads, the oxygen is gradually diminished by organisms that make use of it. So, by sampling the oxygen concentration in deep water at various locations, one can plot the direction of the deep-sea currents.

  Such mapping has shown that one major current flows from the Arctic Ocean down the Atlantic under the Gulf Stream and in the opposite direction, another from the Antarctic up the south Atlantic. The Pacific Ocean gets no direct flow from the Arctic to speak of, because the only outlet into it is the narrow and shallow Bering Strait. Hence, it is the end of the line for the deep-sea flow. That the North Pacific is the dead end of the global flow is shown by the fact that its deep waters are poor in oxygen. Large parts of this largest ocean are therefore sparsely populated with life forms and are the equivalent of desert areas on land. The same may be said of nearly land-locked seas like the Mediterranean, where full circulation of oxygen and nutrients is partly choked off.

  More direct evidence for this picture of the deep-sea currents was obtained in 1957 during a joint British-American oceanographic expedition. The investigators used a special float, invented by the British oceanographer John Crossley Swallow, which is designed to keep its level at a depth of a mile or more and is equipped with a device for sending out short-wave sound waves. By means of these signals, the float can be tracked as it moves with the deep-sea current. The expedition thus traced the deep-sea current down the Atlantic along its western edge.

  THE OCEAN’S RESOURCES

  All this information will acquire practical importance when the world’s expanding population turns to the ocean for more food. Scientific “farming of the sea” will require knowledge of these fertilizing currents, just as land farming requires knowledge of river courses, ground water, and rainfall. The present harvest of seafood—some 80 million tons in 1980—can, with careful and efficient management, be increased (it is estimated) to something over 200 million tons per year, while leaving sea life enough leeway to maintain itself adequately. (The assumption is, of course, that we do not continue our present course of heedlessly damaging and polluting the ocean, particularly those portions of it—nearest the continental shores—that contain and offer human beings the major portion of sea organisms. So far, we are not only failing to rationalize a more efficient use of the sea for food but are decreasing its ability to yield us the quantity of food we harvest now.)

  Food is not the only important resource of the ocean. Sea water contains in solution vast quantities of almost every element. As much as 4 billion tons of uranium, 300 million tons of silver, and 4 million tons of gold are contained in the oceans—but in dilution too great for practical extraction. However, both magnesium and bromine are now obtained from sea water on a commercial scale. Moreover, an important source of iodine is dried seaweed, the living plants having previously concentrated the element out of sea water to an extent that humans cannot yet profitably duplicate.

  Much more prosaic material is dredged up from the sea. From the relatively shallow waters bordering the United States, some 20 million tons of oyster shells are obtained each year to serve as a valuable source of limestone. In addition, 50 million cubic yards of sand and gravel are obtained in similar fashion.

  Scattered over the deeper portions of the ocean floor are metallic nodules that have precipitated out about some nucleus that may be a pebble or a shark tooth. (It is the oceanic analogue of the formation of a pearl about a sand grain inside an oyster.) These are usually referred to as manganese nodules because they are richest in that metal. It is estimated that there are 31,000 tons of these nodules per square mile of the Pacific floor. Obtaining these in quantity would be difficult indeed, and the manganese content alone would not make it worthwhile under present conditions. However, the nodules contain 1 percent nickel, 0.5 percent copper, and 0.5 percent cobalt. These minor constituents make the nodules far more attractive than they would otherwise be.

  And what of the 97 percent of the ocean that is actually water, rather than dissolved material?

  Americans use 95,000 cubic feet of water per person per year, for drinking, for washing, for agriculture, for industry. Most nations are less lavish in their use; but for the world generally, the use is 53,000 cubic feet per person per year. All this water, however, must be fresh water. Sea water, as is, is useless for any of these purposes.

  There is, of course, a great deal of fresh water on Earth in an absolute sense. Less than 3 percent of all the water on Earth is fresh, but that still amounts to about 360 million cubic feet per person. Three-quarters of this is not available for use, to be sure, but is tucked away in the permanent icecaps that cover 10 percent of the planet’s land surface.

  The liquid fresh water on Earth comes to about 85 million cubic feet per person and is constantly replenished by rainfall that amounts to 4 million cubic feet
per person. We might argue that the annual rainfall amounts to 75 times the quantity used by the human race, and that there is therefore plenty of fresh water.

  However, most of the rain falls on the ocean or as snow on the ice pack. Some of the rain that falls on land and remains liquid, or becomes liquid when it grows warmer, runs off to sea without being used. A great deal of water in the forests of the Amazon region is virtually not used by human beings at all. And the human population is steadily growing and is also steadily polluting such fresh water supplies as exist.

  Fresh water is therefore going to be a scarce commodity before long, and humanity is beginning to turn to the ultimate source, the ocean. It is possible to distill sea water, evaporating and then condensing the water itself, and leaving the dissolved material behind, using, ideally, the heat of the sun for the purpose. Such desalination procedures can be used as a fresh-water source and are so used where sunlight is steadily available, or where fuel is cheap, or where needs must. A large ocean liner routinely supplies itself with fresh water by burning its oil in order to distill sea water as well as to run its engines.

  There are also suggestions that icebergs be collected in the polar regions and floated to warm, but arid seaports, where what has survived of the ice can be melted down for use.

  Undoubtedly, however, the best way of utilizing our fresh-water resources (or any resources) is by wise conservation, the reduction to a minimum of waste and pollution, and the cautious limitation of Earth’s human population.

  THE OCEAN DEPTHS AND CONTINENTAL CHANGES

  What about the direct observation of ocean depths? A lone record from ancient times survives (if it can be trusted). The Greek philosopher Posidonius, about 100 B.C., is supposed to have measured the depth of the Mediterranean Sea just off the shores of the island of Sardinia and is said to have come up with a depth of about 1.2 miles.

  It was not until the eighteenth century, however, that scientists began a systematic study of the depths for the purpose of studying sea life. In the 1770s, a Danish biologist, Otto Frederik Muller, devised a dredge that could be used to bring up specimens of such life from many yards beneath the surface.

  One person who used a dredge with particular success was an English biologist, Edward Forbes, Jr. During the 1830s, he dredged up sea life from the North Sea and from other waters around the British Isles. Then, in 1841, he joined a naval ship that was going to the eastern Mediterranean, and there dredged up a starfish from a depth of 450 yards.

  Plant life can live only in the uppermost layer of the ocean, since sunlight does not penetrate more than 80 yards or so. Animal life cannot live except (ultimately) upon plant life. It seemed to Forbes, therefore, that animal life could not long remain below the level at which plants were to be found. In fact, he felt that a depth of 450 yards was probably the limit of sea life and that, below it, the ocean was barren and lifeless.

  And yet, just as Forbes was deciding this, the British explorer James Clark Ross, who was exploring the shores of Antarctica, dredged up life from as deep as 800 yards, well below Forbes’s limit. Antarctica was far away, however; and most biologists continued to accept Forbes’s decision.

  The sea bottom first became a matter of practical interest to human beings (rather than one of intellectual curiosity to a few scientists) when it was decided to lay a telegraph cable across the Atlantic. In 1850, Maury had worked up a chart of the Atlantic sea bottom for purposes of cable laying. It took fifteen years, punctuated by many breaks and failures, before the Atlantic cable was finally laid—under the incredibly persevering drive of the United States financier Cyrus West Field, who lost a fortune in the process. (More than twenty cables now span the Atlantic.)

  But the process, thanks to Maury, marked the beginning of the systematic exploration of the sea bottom. Maury’s soundings made it appear that the Atlantic Ocean was shallower in its middle than on either side. The central shallow region, Maury named Telegraph Plateau in honor of the cable.

  The British ship Bulldog labored to continue and extend Maury’s exploration of the sea bottom. It set sail in 1860; and on board was a British physician, George C. Wallich, who used a dredge and brought up thirteen starfish from a depth of 2,500 yards (nearly 1½ miles). Nor were they starfish that had died and sunk to the sea bottom: they were very much alive. Wallich reported this at once and insisted that animal life could exist in the cold darkness of the deep sea, even without plants.

  Biologists were still reluctant to believe in this possibility; and a Scottish biologist, Charles W. Thomson, went out dredging in 1868 in a ship called Lightning. Dredging through deep waters, he obtained animals of all kinds, and all argument ended. Forbes’s idea of a lower limit of sea life ended.

  Thomson wanted to determine just how deep the ocean is, and set out on 7 December 1872 in the Challenger, remaining at sea for three and a half years for a distance of 78,000 miles altogether. To measure the depth of the oceans the Challenger had no better device than the time-honored method of paying out 4 miles of cable with a weight on the end until it reached the bottom. Over 370 soundings were made in this fashion. This procedure, unfortunately, is not only fantastically laborious (for deep sounding) but is also of low accuracy. Ocean-bottom exploration was revolutionized in 1922, however, with the introduction of echo sounding by means of sound waves; in order to explain how this works, a digression on sound is in order.

  Mechanical vibrations set up longitudinal waves in matter (in air, for instance), and we can detect some of these as sound. We hear different wavelengths as sounds of different pitch. The deepest sound we hear has a wavelength of 22 meters and a frequency of 15 cycles per second. The shrillest sound a normal adult can hear has a wavelength of 2.2 centimeters and a frequency of 15,000 cycles per second. (Children can hear somewhat shriller sounds.)

  The absorption of sound by the atmosphere depends on the wavelength. The longer the wavelength, the less sound is absorbed by a given thickness of air. For this reason, foghorn blasts are far in the bass register so that they can penetrate as great a distance as possible. The foghorn of a large liner like the old Queen Mary sounds at 27 vibrations per second, about that of the lowest note on the piano. It can be heard at a distance of 10 miles, and instruments can pick up the sound at a distance of 100 to 150 miles.

  Sounds also exist deeper in pitch than the deepest we can hear. Some of the sounds set up by earthquakes or volcanoes are in this infrasonic range. Such vibrations can encircle the earth, sometimes several times, before being completely absorbed.

  The efficiency with which sound is reflected depends on the wavelength in the opposite way. The shorter the wavelength, the more efficient the reflection. Sound waves with frequencies higher than those of the shrillest sounds we hear are even more efficiently reflected. Some animals can hear shriller sounds than we can and make use of this ability. Bats squeak to emit sound waves with ultrasonic frequencies as high as 130,000 cycles per second and listen for the reflections. From the direction in which reflections are loudest and from the time lag between squeak and echo, they can judge the location of insects to be caught and twigs to be avoided. They can thus fly with perfect efficiency if they are blinded, but not if they are deafened. (The Italian biologist Lazzaro Spallanzani, who first made this observation in 1793, wondered if bats could see with their ears, and, of course, in a sense, they do.)

  Porpoises, as well as guacharos (cave-dwelling birds of Venezuela), also use sounds for echo-location purposes. Since they are interested in locating larger objects, they can use the less efficient sound waves in the audible region for the purpose. (The complex sounds emitted by the large-brained porpoises and dolphins may even, it is beginning to be suspected, be used for purposes of general communication—for talking, to put it bluntly. The American biologist John C. Lilly investigated this possibility exhaustively with inconclusive results.)

  To make use of the properties of ultrasonic sound waves, humans must first produce them. Small-scale production and use are
exemplified by the dog whistle (first constructed in 1883). It produces sound in the near ultrasonic range which can be heard by dogs but not by humans.

  A route whereby much more could be done was opened by the French chemist Pierre Curie and his brother, Jacques, who in 1880 discovered that pressures on certain crystals produced an electric potential (piezoelectricity). The reverse was also true. Applying an electric potential to a crystal of this sort produced a slight constriction as though pressure were being applied (electrostriction). When the technique for producing a very rapidly fluctuating potential was developed, crystals could be made to vibrate quickly enough to form ultrasonic waves. This was first done in 1917 by the French physicist Paul Langevin, who immediately applied the excellent reflective powers of this short-wave sound to the detection of submarines—though by the time he was done, the First World War was over. During the Second World War, this method was perfected and became sonar (“sound navigation and ranging,” ranging meaning “determining distance”).

  The determination of the distance of the sea bottom by the reflection of ultrasonic sound waves replaced the sounding line. The time interval from the sending of the signal (a sharp pulse) and the return of its echo measures the distance to the bottom. The only thing the operator has to worry about is whether the reading signals a false echo from a school of fish or some other obstruction. (Hence, the instrument is useful to fishing fleets.)

  The echo-sounding method not only is swift and convenient but also makes it possible to trace a continuous profile of the bottom over which the vessel moves, so that oceanographers are obtaining a picture of the topography of the ocean bottom. More detail could be gathered in five minutes than the Challenger could have managed in its entire voyage.

  The first ship to use sonar in this way was the German oceanographic vessel Meteor, which studied the Atlantic Ocean in 1922. By 1925, it was obvious that the ocean bottom was by no means featureless and flat, and that Maury’s Telegraph Plateau was not a gentle rise and fall but was, in fact, a mountain range, longer and more rugged than any mountain range on land. It wound down the length of the Atlantic, and its highest peaks broke through the water surface and appeared as such islands as the Azores, Ascension, and Tristan da Cunha. It was called the Mid-Atlantic Range.