Comet, Revised

by Carl Sagan

  Liquid Water

  The kind of water ice we have described is the most common—those hexagonal crystal lattices characterize snowflakes and icebergs, glaciers and street slush on Earth. The structure is really quite remarkable. Ice has a lower density than liquid water because of the large void volumes or holes in the structure. This is why ice floats on liquid water. This is also why ice expands on freezing. In the liquid (see this page), the water molecules are moving too vigorously to have settled down into the formal ice lattice. They are busy tumbling and colliding, and leaving little in the way of interior voids. But when the temperatures fall, the motion becomes less vigorous and the chemical bonds between adjacent water molecules assert themselves. The hexagonal lattice is formed and a material significantly less dense is formed. The density of liquid water is 1.0 grams per cubic centimeter, but the density of ordinary ice is 0.92 grams per cubic centimeter, a circumstance otherwise virtually unknown in nature. For other substances, the solid is almost always more dense than the liquid and promptly sinks. Floating icebergs and ice-covered rivers will be unheard of on planets where the dominant liquids are ammonia, say, or hydrocarbons such as methane.

  To prevent ice from evaporating directly into vapor, to detain it in an intermediary liquid stage, requires an atmosphere. Collisions with the overlying gas molecules then inhibit evaporation. With no atmosphere, there are no collisions, no such inhibition, and therefore no liquids. Even Mars, with an atmospheric pressure thousands of times that at the surface of a comet, cannot today maintain open bodies of liquid water. There can be no liquid water on a cometary nucleus because, even when its coma is most prominent, a cometary nucleus is still, by Earth standards, surrounded by a high vacuum. You could imagine an interior pocket of the comet in which, if the temperatures were high enough, liquid water could readily form. But if the only source of heat is sunlight from without, the temperatures will get colder the deeper inside the comet you go. Modern speculation about life in comets, discussed later, is closely tied to the possibility of liquid water deep inside the cometary nucleus, perhaps even oceans of underground water.

  It is warmest in the afternoon on Earth rather than exactly at noon when the Sun is highest, because it takes a little time for the ground to warm up. The same is true on the comet, and it is therefore the afternoon side that heats up most and drives the bulk of the vaporized ice into space. Depending on which direction the comet rotates in, this will either accelerate or decelerate its orbital motion.

  Sometimes a comet is structurally weakened—by collision, or by rapid rotation, so it is straining against the cohesive forces—and it splits into two or more pieces. The new frozen volatiles now exposed to sunlight produce still further jets of gas and dust, and some additional small tumbling and darting of the cometary fragments.

  Something similar may even occur in comets that never approach the Sun closely. The most famous case is Comet Schwassmann-Wachmann 1. It lives between the orbits of Jupiter and Saturn and undergoes episodic outbursts, occasionally brightening by a thousand times in just a few days. When there are no such outbursts it appears to be a dark, reddish object, quite similar to asteroids rich in organic matter. One view of the outbursts of Schwassmann-Wachmann 1 is that there are deeply buried repositories of exotic ices being gradually heated by the Sun. Sunlight eventually volatilizes a pocket of ice, and water vapor pours off the surface. Tiny grains of ice and dust, carried along by the gushing water vapor, produce a temporary cloud around the object—a distant coma—that we see only as a temporary brightening. The circular orbit of this comet speaks to a long residence time in this part of the solar system. So why are there any exotic ices still left near the surface? Perhaps this is not a case of exotic ices, but of something else—collision of the comet with boulder-sized objects in its orbit, say, or collisions among the components of a multiple nucleus still undetected directly. The true explanation of the outbursts of Schwassmann-Wachmann 1 remains a mystery. It makes us wonder about other objects in the outer solar system—for example, Chiron, a small world that lives between Saturn and Uranus, or the icy moons of the giant planets. Might one of them someday abruptly wrap itself in clouds and brighten dramatically? (In recent years Chiron has been found to do exactly this.)

  When the German mathematician F. W. Bessel witnessed the jets issuing from the nucleus of Halley’s Comet in 1835, he speculated that a small nongravitational motion of the comet might result. The suggestion languished for 115 years, until Whipple revived it in a modern context. We can today see jets of matter spraying from the nucleus of a warm comet, irregularly turning on and off like the attitude control jets of an interplanetary spacecraft. Every time one of these great fountains goes shooting off into space, the cometary orbit makes a little turn.

  Thus, the dirty ice model in one swoop explains the timing and extent of the formation of cometary comas and tails, the nongravitational motion, and the explosive jetting from cometary nuclei that were noted at least since the time of Thomas Wright. As we shall see, there is now, as well, direct evidence for water ice as the principal constituent of the comets that we observe. Fred Whipple’s “obvious” explanation, in the best tradition of science, produces a windfall of accurate prediction from a modest investment of hypothesis.

  So it looks very much as though comets really are giant iceballs racing around the Sun. How much ice is that? Imagine that you had snowplows stationed all over the Earth whose function it is to sweep up every flake of snow that falls in a year. And then imagine that all this snow is somehow packed into a rough sphere, carried out into space, and put into cold storage. You would then have something like a cometary nucleus ten kilometers across. You could make a hundred ordinary nuclei with this much snow. Put another way, a typical cometary nucleus contains about as much snow as falls each year in Eastern Europe, or the Northern United States. It does not seem so very much.

  An Earthbound astronomer puzzles out the anatomy of a comet. Sheet music cover, 1910. Courtesy Ruth S. Freitag, Library of Congress.

  In a way, Whipple’s snowball hypothesis is a disappointment. Newton and Halley converted comets from terrifying omens to a commonplace of nature, obedient to a divine mandate, unseen but manifest. But if comets are merely snowballs in orbit, is there any mystery left? Or are comets now demoted, becoming prosaic to the point of tedium? Remarkably, the new understanding of comets suggests that they are the key to the origin of the solar system, the origin of life, the origin of us humans, and to the surface characteristics of most of the worlds we know.

  *An exaggeration. The nongravitational forces change the period of Encke’s Comet by no more than one day each orbit. The orbital period is 1200 days. So even to measure the effect requires an error less than 1/1200, or about 0.1 percent, not far short of perfect.

  *But there is, for another comet, evidence that the nucleus is accompanied by a swarm of debris; there may be some life left in the gravel-bank hypothesis, but as an addition to, not a replacement of, the cometary nucleus.

  *The first of these is called acetic acid. The names of the other two are almost unpro-nounceable, and much longer than their formulas.

  *Depending on the local atmospheric pressure, gases can freeze solid without passing through an intermediate liquid stage; this is true for water vapor on Mars or an interstellar grain, but not much on the surface of the Earth, where the atmospheric pressure is large.

  *Ice formed at very low temperatures tends to be amorphous, without the repetitive geometry of the crystal lattice, but there too clathrates form.

  CHAPTER 7

  The Anatomy of Comets:

  A Summary So Far

  There was another sign seen in Heaven, and behold a great red dragon …

  And his tail draweth a third part of the stars in Heaven.

  —THE REVELATION OF ST. JOHN THE DIVINE 12:3–4

  In seeking to understand what comets are, where they come from, and what significance they conceivably might hold for us, we are the beneficiar
ies of thousands of years of patient observation and record-keeping by people all over the planet. An official scribe of Han Dynasty China or Seleucid Babylon, matter-of-factly noting the properties of a comet, helps us millennia later to test Newtonian gravitational theory, or the constancy of the arrival rate of new comets.

  In all of human history, less than a thousand individual comets have been recorded, and only a few hundred have been seen in more than one passage by the Sun and Earth. Pliny wrote that naked-eye comets were visible for between a week and six months, which is still generally true. In most cases the comet is faint, barely visible as a bright smudge in the sky—a wisp of light, seemingly, a small fragment of the Milky Way broken off and on its own. The visibility of such comets greatly improves with a small telescope, or even a pair of binoculars.

  Comets do not streak across the sky; they rise and set with the stars. The notion of streaking stems partly from a confusion with meteors, and partly from the immediate impression given by drawings and photographs: in our everyday experience, objects with such shapes are usually streaking. When we see a picture of a comet some of us are immediately reminded of a woman with long, straight hair being blown back behind her, the reason, as we have said, for the very name comet, derived from the Greek word for hair. But the comet does not live down here, where there is air to blow things back. The comet lives in the nearly perfect vacuum of interplanetary space. And the tail does not always trail behind; instead, after perihelion passage, as the comet leaves the Sun, the tail precedes the nucleus. Something else is going on.

  As a comet approaches the Earth, it typically increases both its brightness and the length of its tail. It may disappear into the Sun’s glare as it negotiates perihelion passage, and then once more is visible—either brighter or dimmer, depending on the relative geometry of Sun, Earth, and comet. The tails of some naked-eye comets have stretched from horizon to zenith.

  Somewhere between the orbits of Jupiter and Mars a bright comet may first appear to Earthbound naked-eye observers as a point of light—a star of fourth or fifth magnitude, surrounded by a perceptible haze. But such apparitions are rare. Comets vary greatly in brightness, of course; most are visible only through large telescopes, some can be seen with the naked eye, and occasionally—once every few years—there is one so prominent as to cause a general stir. About once a human lifetime, on the average, a comet appears that can be seen in the daytime sky, even very close to the Sun. The Great Comet of 1910 (1910 I)—which in people’s memories is sometimes confused with Halley’s Comet which arrived later that year—was such, and was called the Great Daylight Comet.

  Most comets are found by astronomers, professional or amateur. Sometimes, during a total eclipse, a comet is discovered near the Sun, bathed in the light of the solar corona—a comet previously unknown, hitherto invisible, bleached out in the Sun’s glare. After the eclipse, the comet becomes invisible again. But such instances are infrequent.* More often, comets are found in the vicinity of the Sun, as their motion carries them out of the glare, and comets are also discovered far from the Sun; a time exposure at night of a field of sky brings out some faint, nebulous object that is not on the standard charts. Amateurs—who can become ardent about the search for new comets—sometimes systematically scan the heavens in strips, using special telescopes able to see large patches of the sky in a single view. One recent comet was discovered by an amateur astronomer in his living room, gazing out the window with binoculars into the usually unpromising British sky. Some amateurs have found more than a dozen separate comets in a lifetime of dedicated searching.

  With atmospheric pollution and city lights, the practice is now becoming less fashionable, but once there were many people who knew the map of the sky like the back of their hands, and who could tell at a glance—when out on a brisk postprandial constitutional—that there was a new point or smudge of light up there where no star had been seen before. Sometimes exploding stars, novas, are discovered in this way. And sometimes comets. Occasionally a bright comet is first detected with the naked eye by observers who are not astronomers at all. The classic case is the Great Daylight Comet of January 1910, which was discovered by three laborers on the South African railroads. There were then, as in Halley’s time, few astronomers scanning the skies of the Southern Hemisphere.

  Every night, on average, somewhere on Earth, there is at least one astronomer peering through the telescope at a comet. Almost always this is not part of the discovery process, but a segment of a much more intricate research program to understand the nature of comets. An astronomer might be photographing the comet, or passing its light into a spectrometer to uncover something about its composition and motions, or measuring the heat given off by the coma. Most often the discovery is not made by a human being at the eyepiece of a telescope, but from a photograph of the sky, taken through the telescope for quite another purpose.

  What do you look for when you think you’ve discovered a comet far from the Sun? You see a fuzzy patch of light. Might it be an artifact of the photographic emulsion? Take another picture. Could it be some other kind of faint patch of light—a nebula or a distant galaxy—not listed on your chart of the sky? Take another picture. If it moves with respect to the stars at reasonable speeds, then you have probably discovered a comet. If it isn’t fuzzy, it might be an asteroid.

  Naming the Comets

  Comets are often named after their discoverers, as Comet Hale-Bopp or Comet West. There is even a Purple Mountain Observatory Comet, discovered in a time when individual achievement was out of favor in the People’s Republic of China.

  Sometimes comets are named not after their discoverers, but after those who first recognized that the comets seen in two or more apparitions were really the return of the same comet. Halley’s Comet is such a case, as is Encke’s.

  Comets are also designated by a Roman numeral, indicating the sequence of perihelion passages for the comets of a given year—as, 1858 VI, or 1997 I.

  Discoveries of comets by professional astronomers are mainly made when the comet is far from the Sun, either on approach or—more rarely—after perihelion passage. Almost invariably (except when a known comet is being “recovered”) such discoveries are made as an incidental by-product of some very different study. Discoveries by amateur astronomers, in contrast, are most often made when the comet is close to the Sun—within a few hours after sunset or before dawn. Because comets may brighten erratically, a comet that had been undetectable in a large astronomical telescope when it was far from the Sun may sometimes be discovered with a very modest instrument, or even with the naked eye, when it is close to the Sun—as was the case for the Great Daylight Comet of 1910.

  A good fraction of comets with orbits reliably established as periodic are “recovered”—seen on some future approach to the Earth. But how can you be sure that what you’re seeing is the same comet observed years ago? The comet generally has no distinctive identifying characteristics, no regimental tie, no tartan. Nevertheless, there are ways. Following Edmond Halley’s pioneering work, the orbital characteristics of the earlier comet and the present one are compared: the period, eccentricity, distance of perihelion from the Sun, and inclination of the orbit, for example. For the 1986 return of Halley’s Comet astronomers had calculated precisely where the comet should be at every point in its orbit. It was recovered with a large telescope on October 16, 1982, more than three years before perihelion, when it was beyond the orbit of Saturn. The comet was less than 1 percent the apparent size of the Moon from its predicted position.

  Discoveries of comets are sometimes made that remain unconfirmed—usually because the orbital periods are undetermined or too long. Occasionally, short-period comets with well-determined orbits are not recovered in their next apparition. Many such reports are probably not due to an error made by the astronomer, but to a comet, ordinarily too faint to see, which is undergoing an explosive outburst of gas and dust. When the activity is over, the comet returns to obscurity. These unrecover
ed comets serve to remind us that there is a vast population of undiscovered comets.

  From the cumulative weight of astronomical observation, an overall picture of the anatomy of comets has emerged, and even a little of their physiology. We have mentioned various components of this picture already; let us now take stock.

  Comets are by far the largest and most variable objects visible in the solar system.* A tiny nucleus produces a substantial coma and an enormous tail, often much larger than the Sun. But for two comets at the same distance from the Sun, one may have an immense tail, and the other none at all. Following the spirit and precedent of Thomas Wright (Chapter 4), we show the relative scales of a typical cometary nucleus, coma, and tail on this page and this page. When it can be seen at all, the nucleus of a comet appears through the telescope as a starlike point of light. Generally it is a few kilometers across, and yet this tiny ball of ice can generate a visible tail that is longer than the distance between adjacent planetary orbits in the inner solar system. A one-kilometer object with a tail a hundred million kilometers long is like a solitary mote of dust dancing in the sunlight in Washington, D.C., with a tail that reaches Baltimore.

  The components of a comet. Facing page, a coma in the inner solar system is shown next to an image of the Earth. The coma is composed of diffuse gas and fine particles, with the nucleus an insignificant and in this case invisible dot at the center. This page, top, a typical cometary nucleus, a few kilometers across, is shown for scale near the diffuse outer edge of a typical cometary coma. Bottom, a well-developed cometary tail extends from the orbit of Earth to the orbit of Mars. Diagrams by Jon Lomberg/BPS.