Comet, Revised

by Carl Sagan

  Antecedents of Cometary Ice

  Newton had obliquely implied that comets are made mostly of water (see Chapter 17), and Laplace mentioned casually that comets might be made of ice. They did not spell out their arguments, and these suggestions were mainly forgotten. But by the middle of the twentieth century the idea was in the air again. For example, in the Annales d’Astrophysique, in 1948, there is an article by the Belgian astronomer Pol Swings, who says (rough translation) “At large distances from the Sun, all the solids in a comet are at very low temperatures and all the ‘gas’ that they contain, except for hydrogen and helium, must be found in the solid state.” A footnote, however, indicates that this insight earlier appeared in an article by the German astronomer K. Würm, writing in the Mitteilungen der Hamburger Sternwarte Bergedorf, in 1943. But in Würm’s article, the comment is attributed to a remark made informally to Würm by the Czech/German chemist Paul Harteck. In 1942 and ’43 Harteck was busy trying to build an atomic bomb for the Nazis. His comments on cometary ices were a distraction from other, more pressing duties.

  Three molecules of water vapor, each containing an oxygen atom and two smaller hydrogen atoms. The molecules are not linked to their fellows, but are freely moving as part of a gas. They might be molecules of steam released from a teapot, or vaporized off a snowy field on a warm winter’s day. Painting by Jon Lomberg.

  Atoms that are a little rarer in the universe also make chemical combinations. If the silicon atom is symbolized by Si, then ordinary quartz sand is made up of SiO2, silicon dioxide. The molecular oxygen in the air is O2 (as distinguished from an oxygen atom by itself, O). In this way the things of the natural world can be understood in terms of their constituent atoms. We humans are also aggregations of atoms, intricately and wonderfully assembled.

  Now in a molecule like water, the atoms are not stuck onto each other at random positions or angles. An isolated water molecule always has the two small hydrogen atoms stuck onto the bigger oxygen atom at a precise angle, making the molecule look a little like a face with big ears, something like Mickey Mouse (see above). The chemical forces that join atoms together do so according to precise and invariable rules that underlie the beauty and order of nature. These rules are set by the cloud of electrons that surrounds every atom, but for the moment we treat atoms as little impenetrable spheres. If you bring two of them near each other they will tend to combine according to certain definite laws, but if you try to push them too close together they will repel one another. A single isolated water molecule—exhaled from your mouth on a winter’s day, for example—goes bobbing and tumbling off into the air, happily colliding with the other molecules in front of it, and bouncing off them. It does not readily combine with any of them; their constituent atoms are already preoccupied in passionate chemical embraces.

  The molecular structure of ordinary ice. The circles represent the electron clouds of individual atoms, with the tiny atomic nucleus inside. Large circles represent oxygen atoms, smaller circles represent hydrogen atoms. The molecular forces between these atoms assemble them into this hexagonal crystal lattice. We see two of the many successive, parallel planes of linked water molecules that comprise a microscopic fragment of ice. Painting by Jon Lomberg.

  Temperature is nothing more than a measure of the motion of molecules. When the temperature is high, the molecules are in a riotous state, rushing about, tumbling, colliding, bouncing back—a frenzy of activity. As the temperature drops the molecules become more restrained, almost sedate. At sufficiently low temperatures, water molecules connect—they are moving so slowly that the short-range molecular forces can now engage, and adjacent water molecules join weakly with one another. When this happens—on a rainy day, for example—we say that the gas has condensed into liquid water. At lower temperatures still (below the freezing point), the molecules condense not in a helter-skelter order, but in an elegant repetitive pattern called a crystal lattice. This is the hidden structure of ice (see this page). In such a lattice every water molecule is in a specific place, joined to its neighbors.

  In two dimensions, the pattern is hexagonal, like the tiles on a bathroom floor. Every hexagon is made of six oxygen atoms and the accompanying little hydrogen atoms. If you had a superior sort of microscope, still to be invented, you might see such a structure. You could travel a million oxygen atoms to the left or the right, and find exactly the same structure. In three dimensions the crystal lattice is a kind of hexagonal cage. Beyond the oxygen hexagon in front of us is another oxygen hexagon connected by the little hydrogen atoms. Beyond that is another, and so on. You could turn your head almost 120° and not see any discernible difference in the pattern. This hexagonal symmetry on the molecular level carries all the way up to the macroscopic level, and is responsible for the exquisite six-sided symmetry of snowflakes—a truth, oddly enough, first glimpsed by the astronomer Johannes Kepler.

  Evaporating ice. At bottom is a portion of the ice crystal structure, but at the warming top surface (middle of the diagram) the smallest fragments of ice—simple water molecules—are pouring off into the adjacent space. The process is called evaporation or sublimation. Painting by Jon Lomberg.

  If we were to look closely at such a lattice, we would notice that the constituent atoms were not stationary, but instead were vibrating and throbbing in place. If we decreased the temperature, the throbbing would moderate; if we increased the temperature, the throbbing would become more violent. There is a characteristic strength of the chemical bonds that hold the ice crystal lattice together. At a certain temperature the constituent atoms are throbbing so violently that some of the bonds become broken and a small piece of crystal lattice—an isolated water molecule—detaches itself from its fellows and goes tumbling off. Statistically, this occurs on occasion even at low temperatures. As the temperature increases, it occurs more often. At high enough temperatures the throbbing motion becomes so violent that the upper layers of ice become disengaged and large numbers of individual molecules go gushing off. If this happens on a cometary nucleus, the water molecules then find themselves in nearby interplanetary space. The process is called, variously, evaporation, vaporization, volatilization, or sublimation. Fundamentally, it is a change from water in the solid state, called ice, directly to water in the gaseous state, called vapor, without experiencing an intermediate liquid state.

  If we had a piece of ice in a closed container and heated it up, at the characteristic temperature the water molecules would start spewing off in earnest. But they could not escape to space; they would bounce off each other as well as the walls of our chamber, and eventually they would all return to the surface of the ice, where they tend to stick. An equilibrium would be established between molecules of water coming off the ice and molecules of water bouncing back to it. Under these circumstances the rate at which the ice would be transformed into water vapor would be slow, and it is such circumstances with which we are familiar on Earth. An ice cube in a covered jar disappears more slowly than one in the open air. But on a comet there is no air, only a nearly perfect vacuum. So once the ice of the comet is sufficiently warmed, it begins to lose water molecules to space quickly, and forever.

  A molecule of methane sits trapped in one of the hexagonal cages of an ice crystal lattice. The carbon atom is shown with white outlines, attached to four smaller hydrogen atoms, comprising methane, CH4. Ices with such trapped molecules inside them are called clathrates. Painting by Jon Lomberg.

  Water ice isn’t the only kind of ice there is. If you take carbon monoxide gas or methane gas and sufficiently cool it, it will also form a cold, white crystalline solid. The temperature at which these gases form ices is called the freezing point, a different temperature for each material.* The freezing points of some common gases are shown in the adjacent table. The fact that water molecules crystallize at a comparatively high temperature is due to the strength of its chemical bonds; methane ice or carbon monoxide ice falls to pieces at temperatures where water ice is quite stable and happy
.

  Freezing Points of Ices

  Water, H2O 0°C.

  Hydrogen Cyanide, HCN –14°

  Carbon Dioxide, CO2 –57°

  Ammonia, NH3 –78°

  Formaldehyde, HCHO –92°

  Methane, CH4 –182°

  Carbon Monoxide, CO –199°

  Nitrogen, N2 –210°

  Temperatures are given in degrees Centigrade, or Celsius.

  Now imagine some mixture of methane, ammonia, water, carbon monoxide, and other gas molecules, but with water in excess, as the general cosmic abundances suggest should be typical. You have to be about as close to a sun-like star as the Earth is to the Sun for water to be in the liquid state. Most of the universe is of course far from stars, and therefore well below the freezing point of water. On some tiny grain of dust out beyond Saturn or between the stars, the temperatures are very low, and a molecule of water hitting the grain will stick. As the grain passes over millions of years through the thin interstellar gas, it grows, and the crystal lattice bit by bit extends itself in all directions. As it does, other sorts of molecules may become trapped in the cage—a molecule of methane, perhaps, or ammonia, or something else (see this page). This kind of water ice structure, in which foreign molecules are trapped inside the cage, is called a clathrate.* The trapped molecule is not chemically combined with the ice, but merely imprisoned physically. You can see there is room for only about one big atom in the cage, and so methane clathrates tend to have around six water molecules for every methane molecule. In the vaporization of a water clathrate, the imprisoned molecules are released to space at the same time that the ice crystal lattice peels off.

  If this grain is growing over millions of years it may be composed largely of water ice, because there is so much more water than anything else that will condense onto the grain. But other materials should be trapped as clathrates, and other ices should form on its surface—patches of CH4 ice or CO ice or CO2 ice, as well as much more complex molecules. If the grains continue to grow and collide with one another, building structures of increasing size, eventually something like a small cometary nucleus will evolve. It will not be purely ice; there are many other kinds of material available, some of which we will mention later. But for now let us imagine the cometary nucleus as composed only of ices, and then picture it plummeting into the inner solar system. The temperature at its surface slowly warms. Because ice is not a good conductor of heat, the interior of the cometary nucleus remains for a long time at the temperature of the interstellar cold. But the outside becomes steadily warmer. Eventually it gets so hot that the chemical bonds holding the ice together begin to break, and the outer layers of ice go spewing off the comet into space.

  States of matter in the present solar system at various distances from the Sun. Concentric orbits are shown, counting outward from the Sun, of the planets Mercury, Venus, Earth, Mars, Jupiter, and Saturn. For three kinds of materials—silicate rocks, water, and methane—the region in the solar system is shown in which they are gaseous (cloud), liquid (droplet), and solid (cube). Silicates are vaporized interior to the orbit of Mercury. Water is liquid in the vicinity of the Earth, and frozen at greater distances from the Sun than Mars. Methane can be liquid as far as the orbit of Saturn, but is solid at still greater distances from the Sun. Thus a mix of solid methane, water, and silicates making up a comet and entering the solar system from afar finds its methane first evaporating near the orbit of Saturn, water in earnest near the orbit of Mars, and silicates inside the orbit of Mercury. Diagram by Jon Lomberg/BPS.

  Different ices experience this violent vaporization at different temperatures, and therefore at different distances from the Sun. As the comet crosses the orbit of Neptune, a patch of pure methane ice is feebly warmed by the approaching Sun; as chemical bonds are broken, a puff of methane gas is lost to space. A patch of ammonia ice would be lost as the comet crossed the orbit of Saturn. Carbon dioxide ice would begin vaporizing in earnest somewhere between the orbits of Saturn and Jupiter. Ordinary water ice would not begin to vaporize significantly until the comet neared the asteroid belt, between the orbits of Jupiter and Mars.

  But it is precisely in the vicinity of the asteroid belt that most cometary comas are observed to form. This in itself is evidence that water ice is a principal volatile constituent of the comets, and that water vapor and its degradation products are major components of the comas of comets. Much more direct evidence for cometary water now exists, as we shall see.

  Occasionally, we do see comets outgas, forming at least temporary comas and even tails, when they are exterior to the asteroid belt. It is tempting to attribute such activity to the vaporization of other kinds of ices, so-called exotic ices such as CH4 or CO2. But the farther from the Sun that these outbursts occur, the less likely it is that they will be noticed by the handful of astronomers on Earth. Imagine a comet with a surface composed of patches of different sorts of ices—water ice, methane ice, ammonia ice, carbon dioxide ice—and in an orbit that at first brings it only to the vicinity of Uranus, say, or Neptune. Each perihelion passage it will outgas methane ice (and, if any, nitrogen ice and carbon monoxide ice). But these puffs of gas, even if on a significant scale, will remain unnoticed and unrecorded on Earth. After many perihelion passages the methane—at least in the outer layers of the cometary nucleus—will have been entirely lost to space: the comet will have become devolatilized in methane. Each time it loses methane it becomes, relatively speaking, richer in water. If now such a comet is perturbed—by a close approach with the planet Neptune, say—into the inner solar system, it will lose ammonia, if it has any, when passing Saturn. But it will be most easily detectable when it comes interior to the orbit of Mars, heats up sufficiently that the strong lattice structure of ice becomes disrupted and an enormous cloud of the abundant volatile water comes pouring out. Thus, the prominence of water ice in comets is due to three factors: (1) the high cosmic abundance of water; (2) the possibility of loss of other volatiles in earlier incarnations of the comet’s orbit; and (3) the fact that the water comes spewing off only in the inner solar system when the comet is close enough for Earth-bound observers to see.

  Whipple and others showed that if a comet is made of ices, it would be able to supply copious quantities of molecules and small particles to form the coma and the tail. A cometary nucleus might lose a meter of material or more during each perihelion passage at 1 A.U. If it started out with a radius of one kilometer, its substance would all be spent after a thousand perihelion passages. All that would be left would be materials like the involatile mineral grains—some of which, sooner or later, might be swept up by the Earth, producing a meteor shower. Think of two comets, both with perihelia near the Earth’s orbit, but with different aphelia, and different periods of revolution around the Sun. Suppose one of them is a short-period comet and takes five years per orbit. Then from the moment it arrives in the inner solar system to the moment it is entirely vaporized and converted into meteors is five thousand years. The long-period comet, with a period of a hundred years, say, takes instead a hundred thousand years to lose its volatiles. These are typical lifetimes to be expected for comets, provided there is a fresh ice surface exposed directly to sunlight each perihelion passage. The layers of the cometary onion peel off near every perihelion until eventually there is nothing left. Therefore, the population of short-period comets must be resupplied from a more distant repository of comets, just as Laplace and others had calculated (see Chapter 5).

  Molecular structure of liquid water. The water molecules are not arranged in a rigid crystal lattice, but are randomly oriented and free to move. Painting by Jon Lomberg.

  Whipple also realized that his dirty ice model could, in a most natural way, explain the strange nongravitational motion of some comets like Encke. Take an ordinary air-filled toy balloon, tie the nozzle closed, and let it fall. Its downward trajectory is slow and steady. But if you take the same balloon and hold it closed above your head, the nozzle squeezed between th
umb and forefinger, when you let go it briefly darts across the room making sudden starts and turns and occasional rude noises. This is the rocket effect. When the air rushes out of the nozzle, the balloon darts in the opposite direction. The reason is called Newton’s Third Law of Motion: for every action there is an equal and opposite reaction. A rocket works on exactly the same principle. The exhaust blasts down on the launch pad and the rocket lifts up into the sky. The rocket exhaust does not push against the ground or against anything at all; rockets work equally well—indeed, better—in the vacuum of space. Another familiar example is the recoil of a rifle—the bullet goes forward, and the stock drives back into your shoulder.

  Akin to the air in the balloon, the fuel in the rocket, and the bullet in the gun is the ice in the comet. Imagine an iceberg tumbling toward the Sun, its surface covered with patches of rocky material, interspersed with the more volatile ices. The surface of the comet is heated as it approaches the Sun. Some of the ice warms up and vaporizes. You can imagine a little gush of methane or ammonia gas into space—perhaps uncovering some deeper vein of the material, or perhaps only some rocky matrix material. But these jets of gas do not arise uniformly from all over the surface of the cometary nucleus at once. As a patch of methane ice vaporizes (action) at the orbital distance of Neptune, the orbit of the comet shudders slightly (reaction). Closer to the Sun, patches of ammonia or carbon dioxide can produce similar rocket effects. Detailed studies show that the tiny amount of nongravitational motion in the short-period comets can readily be explained by the rocket effect from subliming veins of water ice on the surface of the comet.