by Isaac Asimov
Hydrogen is not, or should not, be unique as far as radio waves are concerned. Every different atom, or combination of atoms, is capable of emitting characteristic radio-wave radiation or of absorbing characteristic radio-wave radiation from a general background. Naturally, then, astronomers sought to find the telltale fingerprints of atoms other than the supremely common hydrogen.
Almost all the hydrogen that occurs in nature is of a particularly simple variety called hydrogen-1. There is a more complex form, which is deuterium or hydrogen-2. The radio-wave radiation from various spots in the sky were combed for the wavelengths that theory predicted. In 1966, it was detected, and the indications are that the quantity of hydrogen-2 in the universe is about 5 percent that of hydrogen-1.
Next to the varieties of hydrogen, as common components of the universe, are helium and oxygen. An oxygen atom can combine with a hydrogen atom to form a hydroxyl group. This combination would not be stable on earth, for the hydroxyl group is very active and would combine with almost any other atom or molecule it encountered. It would, notably, combine with a second hydrogen atom to form a molecule of water. In interstellar space, however, where the atoms are spread so thin that collisions are few and far between, a hydroxyl group, once formed, would persist undisturbed for long periods of time—as was pointed out in 1953 by the Soviet astronomer I. S. Shklovskii.
Such a hydroxyl group would, calculations showed, emit or absorb four particular wavelengths of radio waves. In October 1963, two of them were detected by a team of radio engineers at Lincoln Laboratory of M.I.T.
Since the hydroxyl group is some 17 times as massive as the hydrogen atom alone, it is more sluggish and moves at only one-fourth the velocity of the hydrogen atom at any given temperature. In general, movement blurs the wavelengths so that the hydroxyl wavelengths are sharper than those of hydrogen. Its shifts are easier to determine, and it is easier to tell whether a gas cloud, containing hydroxyl, is approaching or receding.
Astronomers were pleased, but not entirely astonished, at finding evidence of a two-atom combination in the vast reaches between the stars. Automatically, they began to search for other combinations, but not with a great deal of hope. Atoms are spread so thin in interstellar space that there seemed to be only a remote chance of more than two atoms coming together long enough to form a combination. The chance that atoms less common than oxygen (such as those of carbon and nitrogen, which are next most common of those able to form combinations) would be involved seemed out of the question.
But then, beginning in 1968, came the real surprises. In November of that year, they discovered the telltale radio-wave fingerprints of water molecules (H2O). Those molecules were made up of 2 hydrogen atoms and 1 oxygen atom—3 atoms altogether. In the same month, even more astonishingly, ammonia molecules (NH3) were detected. These were composed of 4-atom combinations: 3 atoms of hydrogen and 1 of nitrogen.
In 1969, another 4-atom combination, including a carbon atom, was detected. This was formaldehyde (H2CO).
In 1970, a number of new discoveries were made, including the presence of a 5-atom molecule, cyanoacetylene, which contained a chain of 3 carbon atoms (HCCCN) and methyl alcohol, with a molecule of 6 atoms (CH3OH).
In 1971, the 7-atom combination of methylacetylene (CH3CCH) was detected; and by 1982, a 13-atom combination was detected. This was cyano-decapenta-yne, which consists of a chain of 11 carbon atoms in a row, with a hydrogen atom at one end and a nitrogen atom at the other (HC11N).
Astronomers found themselves with a totally new, and unexpected, subdivi sion of the science before them: astrochemistry.
How those atoms come together to form complicated molecules, and how such molecules manage to remain in being despite the flood of hard radiation from the stars, which ordinarily might be expected to smash them apart, astronomers cannot say. Presumably these molecules are formed under conditions that are not quite as empty as we assumed interstellar space to be perhaps in regions where dust clouds are thickening toward star formation.
If so, still more complicated molecules may be detected, and their presence may revolutionize our views on the development of life on planets, as we shall see in later chapters.
Chapter 3
* * *
The Solar System
Birth of the Solar System
However glorious and vast the unimaginable depths of the universe, we cannot remain lost in its glories forever. We must return to the small family of worlds within which we live. We must return to our sun—a single star among the hundreds of billions that make up our galaxy—and to the worlds that circle it, of which Earth is one.
By the time of Newton, it had become possible to speculate intelligently about the creation of Earth and the solar system as a separate problem from the creation of the universe as a whole. The picture of the solar system showed it to be a structure with certain unifying characteristics (figure 3.1).
Figure 3.1. The solar system, drawn schematically, with an indication of the hierarchy of planets according to relative size.
1. All the major planets circle the sun in approximately the plane of the sun’s equator. In other words, if you were to prepare a three-dimensional model of the sun and its planets, you would find it could be made to fit into a very shallow cake pan.
2. All the major planets circle the sun in the same direction—counterclockwise if you were to look down on the solar system from the direction of the North Star.
3. Each major planet (with some exceptions) rotates around its axis in the same counterclockwise sense as its revolution around the sun, and the sun itself also rotates counterclockwise.
4. The planets are spaced at smoothly increasing distances from the sun and have nearly circular orbits.
5. All the satellites, with some exceptions, revolve about their respective planets in nearly circular orbits in the plane of the planetary equator and in a counterclockwise direction.
The general regularity of this picture naturally suggested that some single process had created the whole system.
What, then, is the process that produced the solar system? All the theories so far proposed fall into two classes: catastrophic and evolutionary. The catastrophic view is that the sun was created in single blessedness and gained a family, at some comparatively late stage in its history, as the result of some violent event. The evolutionary ideas hold that the whole system, sun and planets alike, came into being in an orderly way at the very start.
In the eighteenth century, when scientists were still under the spell of the Biblical stories of such great events as the Flood, it was fashionable to assume that the history of the earth was full of violent catastrophes. Why not one supercatastrophe to start the whole thing going? One popular theory was the proposal of the French naturalist Georges Louis Leclerc de Buffon, in 1745, that the solar system had been created out of the debris resulting from a collision between the sun and a comet.
Buffon, of course, implied a collision between the sun and another body of comparable mass. He called the other body a comet for lack of another name. We now know comets to be tiny bodies surrounded by insubstantial wisps of gas and dust, but Buffon’s principle would remain if we called the colliding body by some other name; and in later times, astronomers returned to his notion. To some, though, it seemed more natural, and less fortuitous, to imagine a long-drawn-out and noncatastrophic process as occasioning the birth of the solar system. This would somehow fit the majestic picture Newton had drawn of natural law governing the motions of the worlds of the universe.
Newton himself had suggested that the solar system might have been formed from a thin cloud of gas and dust that slowly condensed under gravitational attraction. As the particles came together, the gravitational field would become more intense, the condensation would be hastened, and finally the whole mass would collapse into a dense body (the sun), made incandescent by the energy of the contraction.
In essence, this is the basis of the most popular theories of the origin of th
e solar system today. But a great many thorny problems had to be solved to answer specific questions. How, for instance, could a highly dispersed gas be brought together by the extremely weak force of gravitation? In recent years, astronomers have proposed that the initiating force might be a supernova explosion. Imagine that a vast cloud of dust and gas which has already existed, relatively unchanged, for billions of years, happens to have moved into the neighborhood of a star that has just exploded as a supernova. The shock wave of that explosion, the vast gust of dust and gas that forces its way through the nearly quiescent cloud I have mentioned compresses that cloud, thus intensifying its gravitational field and initiating the condensation that results in the formation of a star.
If this is the way the sun was created, what about the planets? Where did they come from? The first attempts at an answer were put forward by Immanuel Kant in 1755 and independently by the French astronomer and mathematician Pierre Simon de Laplace in 1796. Laplace’s picture was the more detailed.
As Laplace described it, the vast, contracting cloud of matter was rotating to start with. As it contracted, the speed of its rotation increased, just as a skater spins faster when he pulls in his arms. (This effect is due to the conservation of angular momentum: since angular momentum is equal to the speed of motion times the distance from the center of rotation, when the distance from the center decreases the speed of motion increases in compensation.) And as the rotating cloud speeded up, according to Laplace, it began to throw off a ring of material from its rapidly rotating equator, thus removing some of the angular momentum. As a result, the remaining cloud slowed down; but, as it contracted further, it again reached a speed at which it threw off another ring of matter. So the coalescing sun left behind a series of rings—doughnut-shaped clouds of matter. These rings, Laplace suggested, slowly condensed to form the planets; and along the way, they themselves threw off small rings that formed their satellites.
Because, by this view, the solar system began as a cloud, or nebula, and because Laplace, as an example, pointed to the Andromeda Nebula (not then known to be a vast galaxy, but thought to be a spinning cloud of dust and gas), this suggestion became known as the nebular hypothesis.
Laplace’s nebular hypothesis seemed to fit the main features of the solar system very well—and even some of its details. For instance, the rings of Saturn might be satellite rings that had failed to coagulate. (Put all together, they would indeed form a satellite of respectable size.) Similarly, the asteroids, circling around the sun in a belt between Mars and Jupiter, might be products of sections of a ring that had not united to form a planet. And when Helmholtz and Kelvin worked up theories attributing the sun’s energy to its slow contraction, that, too, seemed to fit right in with Laplace’s picture.
The nebular hypothesis held the field through most of the nineteenth century. But apparently fatal Haws began to appear well before its end. In 1859, James Clerk Maxwell, analyzing Saturn’s rings mathematically, showed that a ring of gaseous matter thrown off by any body could only condense to a collection of small particles like the rings of Saturn; it would never form a solid body, because gravitational forces would pull the ring apart before such a condensation materialized.
The problem of angular momentum also arose. It turned out that the planets, making up only a little more than 0.1 percent of the mass of the whole solar system, carry 98 percent of its total angular momentum! Jupiter alone possesses 60 percent of all the angular momentum of the solar system. The sun, then, retains only a tiny fraction of the angular momentum of the original cloud. How did almost all of the angular momentum get shoved into the small rings split off the nebula? The problem is all the more puzzling since, in the case of Jupiter and Saturn which have satellite systems that seem like miniature solar systems and have, presumably, been formed in the same way, the central planetary body retains most of the angular momentum.
By 1900, the nebular hypothesis was so dead that the idea of any evolutionary process at all seemed discredited. The stage was set for the revival of a catastrophic theory. In 1905, two American scientists, Thomas Chrowder Chamberlin and Forest Ray Moulton, using a better term than comet, explained the planets as the result of a near collision between our sun and another star. The encounter pulled gaseous matter out of both suns, and the clouds of material left in the vicinity of our sun afterward condensed into small planetesimals, and these into planets. This is the planetesimal hypothesis. As for the problem of angular momentum, the British scientists James Hopwood leans and Harold Jeffreys proposed, in 1918, a tidal hypothesis, suggesting that the passing sun’s gravitational attraction had given the dragged-out masses of gas a kind of sidewise yank (put “English” on them, so to speak) and thus imparted angular momentum to them. If such a catastrophic theory were true, then planetary systems would have to be extremely scarce. Stars are so widely spaced that stellar collisions are 10,000 times less common than are supernovae, which are themselves not common. It is estimated that, in the lifetime of the galaxy, there has been time for only ten encounters of the type that would produce solar systems by this theory.
However, these initial attempts at designing catastrophes failed when put to the test of mathematical analysis. Russell showed that, in any such near collision, the planets would have to end up thousands of times as far from the sun as they actually are. Furthermore, attempts to patch up the theory by imagining a variety of actual collisions, rather than near misses, had little Illeecss. During the 1930s, Lyttleton speculated about the possibility of a three-star collision, and later Hoyle had suggested that the sun had had a companion that had “gone” supernova and left planets as a legacy. In 1939, however, the American astronomer Lyman Spitzer showed that any material ejected from the sun under any circumstances would be so hot that it would not condense into planetesimals but would merely expand into a thin gas. That seemed to end all thought of catastrophe (although, in 1965, a British astronomer, M. M. Woolfson, suggested that the sun may have drawn its planetary material from a very diffuse, cool star, so that extreme temperatures need not be involved).
And so, after the planetesimal theory had come to a dead end, astronomers returned to the evolutionary idea and took another look at Laplace’s nebular hypothesis.
By that time, their view of the universe had expanded enormously. They now had to account for the formation of galaxies, which called for much bigger clouds of gas and dust than Laplace had envisaged as the parent of the solar system. And it now appeared that such vast collections of matter would experience turbulence and would break up into eddies, each of which could condense into a separate system. In 1944, the German astronomer Carl F. von Weizsacker made a thorough analysis of this idea. He calculated that the largest eddies would contain enough matter to form galaxies. During the turbulent contraction of such an eddy, subeddies would develop. Each subeddy would be large enough to give birth to a solar system (with one or more suns). On the outskirts of the solar eddy itself, subsubeddies might give rise to planets. Thus, at junctions where subsubeddies met, moving against each other like meshing gears, forming dust particles would collide and coalesce, first planetesimals and then planets (figure 3.2).
Figure 3.2. Carl F. von Weizsacker’s model of the origin of the solar system. His theory holds that the great cloud from which it was formed broke up into eddies and subeddies which then coalesced into the sun, the planets, and their satellites.
The Weizsiicker theory, in itself, did not solve the matter of the angular momentum of the planets any more than had the much simpler Laplacian version. The Swedish astrophysicist Hannes Alfven took into account the magnetic field of the sun. As the young sun whirled rapidly, its magnetic field acted as a brake, slowing it up, and the angular momentum was passed on to the planets. Hoyle elaborated on this notion so that the Weizsacker theory, modified to include magnetic as well as gravitational forces, seems the best one yet to account for the origin of the solar system.
The Sun
The sun is clearly the sourc
e of light, of heat, and of life itself on Earth, and even prehistoric humanity must have deified it. The Pharaoh, Ikhnaton, who came to the Egyptian throne in 1379 B.C., and was the first monotheist we know of, considered the sun to be the one god. In medieval times, the sun was the symbol of perfection and, though not considered to be itself a god, was certainly taken as representing the perfection of the Almighty.
The ancient Greeks were the first to get a notion of its actual distance, and Aristarchus’ observations showed that it must be several million miles away at the least and thus, to judge by its apparent size, that it must be larger than the Earth. Mere size, however, was not impressive in itself, since it was easy to suppose that the sun was merely a vast ball of insubstantial light.
Not till Newton’s time did it became obvious that the sun has to be not only larger but much more massive than the Earth, and that the Earth orbits around the sun precisely because the former is bound by the latter’s intense gravitational field. We now know that the sun is about 93,000,000 miles from Earth, and that it is 865,000 miles in diameter, or 110 times the diameter of Earth. Its mass is 330,000 times that of Earth and, indeed, is 745 times that of all the planetary material put together. In other words, the sun contains about 99.86 percent of all the matter in the solar system and is overwhelmingly its chief member.
Yet we must not allow sheer size to overimpress us. It is certainly not a perfect body, if by perfection we mean (as the medieval scholars did) that it is uniformly bright and spotless.
Toward the end of 1610, Galileo used his telescope to observe the sun , during the sunset haze and saw dark spots on the sun’s disk every day. By observing the steady progression of the spots across the surface of the sun and their foreshortening as they approached the edge, he decided that they were part of the solar surface and that the sun was rotating on its axis in a little over twenty-five earth-days.
