Since comets mainly live far from the planets, in the interstellar realm, and may be made ultimately of interstellar grains, it is not unreasonable that they should be made of interstellar stuff. If we assume that comets are composed simply of interstellar silicates and ices—frozen H2O, CH4, NH3, CO2, and so on—and allow molecular fragments to come off under solar radiation, calculations show insufficient C3 and CN produced relative to other molecules. Silicates and ices alone cannot explain all the organic fragments discovered by William Huggins. But if it is assumed that the comets ultimately derive from organic-rich interstellar matter, then the molecules detected in the comas and tails of comets can be understood.
Cometary silicates are probably intimately mixed with—perhaps coated by—complex organic compounds. The relative abundance of atoms in comets, so far as is known, is quite like that in the interstellar grains and gas. There is less carbon detected in the spectra of comets than there is—again in terms of relative abundances—in the spectra of interstellar grains. One possible explanation is a large proportion of complex organic molecules (or just plain carbon) in the cometary nucleus which are involatile or which do not produce accessible spectral features. If this interpretation is correct, comets might be as much as 10 to 25 percent organics. Only a few percent of dark organics would suffice to darken and redden the cometary snows sufficiently to be in agreement with observation. But the best evidence for organic matter and carbon in comets is obtained from spacecraft flybys and from recovered small pieces of cometary matter, a subject we treat below.
The picture that emerges from the work of Huggins and his successors at the spectroscope is this: A comet is a snowball filled with small mineral grains and complex organic matter. The organic molecules are distributed throughout the comet, although they may be concentrated in the surface layers. The amount of organics is at least a few percent—perhaps as much as 10 to 25 percent—enough to darken dramatically and somewhat redden the snows. This chemistry is similar to what we know of the molecules between the stars, and is hard to understand unless the comet has been formed out of interstellar matter. Thus, more than any other body we have access to, comets may be emissaries from the void between the stars.
CHAPTER 9
Tails
Hung by the heavens with black, yield day to night!
Comets, importing change of time and states,
Brandish your crystal tresses in the sky …
—WILLIAM SHAKESPEARE, HENRY VI, PART I, I, I
The tails of comets appear to be composed of the most volatile molecules which the heat of the Sun raises from their surface and by the impulsion of his rays banishes to an indefinite distance … The different volatility, size, and density of the molecules must needs produce considerable differences in the curves which they describe: hence arise the great varieties of form, length, and breadth observed in the tails of comets. If we suppose these effects combine with others which may result from a movement of rotation in the comet … we may partly conceive the reason of the singular phenomena represented by the nebulosities in tails of comets.
—PIERRE SIMON, MARQUIS DE LAPLACE, THE SYSTEM OF THE WORLD, PARIS, 1799
This is a chapter about nothing, or something closer to nothing than anything in our everyday life. A cubic centimeter is the volume of a sugar cube. If you select a cubic centimeter of air in front of your nose and look extremely closely, you would find thirty billion billion molecules, all very small, vigorously running into one another. If, on the other hand, you were to make an inventory of the atoms or molecules in a cubic centimeter of a comet’s tail, you would find a thousand or less—hardly any at all. A bright comet tail is much nearer to a perfect vacuum than the best laboratory vacuum our technology can produce on Earth. But even with very few molecules in a cubic centimeter of comet tail, there are a great many cubic centimeters. It is sometimes said that, like a genie in the Arabian Nights, a cometary tail stretching from world to world can be bottled up in a brass lamp. Newton wrote, “a gaseous comet [i.e., the tail alone], with a radius a thousand millions of miles in length, if submitted to the same degree of condensation as the earth, could be easily kept in a good-sized thimble.” But even with only one molecule per cubic centimeter, the enormous tail Newton imagined would require a thimble three kilometers across to contain it. “Good-sized,” indeed.
The tail is almost perfectly transparent: when it passes in front of a bright star you can always see the star through it. But still, it may seem astonishing that the air in the room should be transparent while the near vacuum in the comet tail is visible to the naked eye. The air is a something; we can feel it readily enough in a stiff breeze. The comet tail is very close to nothing. How can the something be invisible when the nothing is clearly seen? The key to answering this question is to remember that we see the comet against a black sky, while the daylight sky is almost uniformly illuminated. Newton did a very difficult experiment, remembering the brightness of a comet and comparing it with the brightness seen in a laboratory experiment:
Nor is the brightness of the tails of most comets ordinarily greater than that of our air, an inch or two in thickness, reflecting in a darkened room the light of the sunbeams let in by a hole of the window shutter.
A great many widely separated particles reflect light as well as the same number of particles compressed by the overlying atmosphere. A little bit of air illuminated by sunlight and viewed against a black background is as bright as a comet tail in the night sky. Still, what is it that we see when we look at the tail of a comet?
If you are fortunate enough to be standing under a clear sky with a comet readily visible above you, note the orientation of the comet’s tail with respect to the Sun. If it is just before sunrise, note how the tail streams away from the glimmer of dawn on the eastern horizon. If it is just after sunset, note how the tail points away from the Sun, now located just beneath the western horizon. And if you are fortunate enough to witness a great daylight comet, it will then be entirely obvious that the comet tail points away from the Sun. There are no significant exceptions to this rule. Although from comet to comet there is some variation in the exact angle the comet’s tail makes with a straight line from comet to Sun, this regularity is as invariable as the observation that the horns of the Moon’s crescent always point away from the Sun.
Comet tails point away from the Sun whether the comet is approaching the Sun or receding from it, a conclusion apparently first drawn by Chinese astronomers during the apparition of Halley’s Comet in the year 837. After perihelion passage the comet flies tail first out of the solar system. The tails of comets are much more like the effluvia from an industrial smokestack blown back on a windy day than the long hair of a bicyclist flowing behind her as she coasts down a hill on a windless day: it is not the motion of the comet through some resisting gas that determines the orientation of the tails, but rather something like a wind blowing out from the Sun.
There are two kinds of cometary tails: long, straight, faintly blue tails, pointing almost perfectly straight back from the Sun; and usually shorter, curved, faintly yellow tails. Before their nature was understood, they were called Type I and Type II tails, respectively (see this page). These designations are still in use. The Type II tails are yellow because they reflect sunlight back to us; but the Type I tails give off a blue light of their own, although they are commonly not very prominent in visible light. A comet may have either type, or neither or both types of tails at a given time. The Type I tails often display an intricate dancing pattern of straight ray streamers, each ray narrower than the diameter of the Moon, but perhaps ten million kilometers long.
Photographs of Halley’s Comet near its 1910 perihelion passage, located in the actual orbital positions and proper orientations. Diagram by Jon Lomberg/BPS.
The thin straight Type I tails and the curved Type II tails of Donati’s Comet over Paris, October 5, 1858. From Amédée Guillemin’s Les Comètes (Paris, 1875).
Type I tails are variabl
e—not only from comet to comet, but from hour to hour, day to day, week to week, within the same comet. Like lizards, comets can grow new tails. Comets are quick-change artists. In 1908, for example, Comet Morehouse—sizable fragments of which were reported to have left the nucleus for the tail, where they emitted their own subsidiary tails—astonished the astronomers gathered at a conference in Oxford:
The formation of the tail seems to be intermittent rather than continuous. There seem to be at intervals convulsions or explosions in the nucleus, producing big bunches or lumps of tail which travel away and leave the comet with a small tail for a time … the signs of a coming convulsion can be recognized.
A young British astronomer named Arthur Eddington used the newly perfected lantern slide in a talk on Comet Morehouse:
Here is the same comet a day later. Everything is altered completely; you cannot point to a feature in the tail on this photograph and say that it corresponds to a certain feature in the previous photograph, and that one has changed into the other; you cannot say that this is the tail of the previous day modified. As far as can be judged the tail is an entirely new one …
Type I tails sometimes exhibit “knots”—small condensations of matter brighter than their surroundings. Knots are sometimes observed to accelerate down the tail away from the Sun. In talking about Comet Morehouse, Eddington was clearly puzzled by the acceleration turning on and off capriciously. By taking a quick succession of photographs you can measure how fast the knots move. Their speed ranges up to 250 kilometers per second (540,000 miles an hour), or more, and their acceleration (which you would feel if you were moving with the knot) can be as high as 1 g, the acceleration that the Earth’s gravity imparts to falling objects. There can also be knots with very little speed or acceleration. The motion of knots in comet tails is as unpredictable as the weather.
The Comet of July 1819 and Signora Bietta
Oh what a fix! What a damned fix
I got into, if I’d only known, my husband and me!
I’m two weeks late with my …
So that … you know, Mrs. Bietta!
And you know what? All this trouble on account of the Comet
With that tail that amazed everyone so!
The same thing also happened to Amelli,
And to Gina, and to Bina, and to Babetta.
And Nunziada’s a full month late,
You know what a little fool she is:
Just imagine, that sneaky Comet!
And do you remember? We were there
laughing at that enormous tail
and playing under it! Can it be?
—ANONYMOUS NINETEENTH-CENTURY POEM,
TRANSLATED FROM THE ITALIAN BY GINA PSAKI
Visible light spectroscopy of the Type II tail shows light from the Sun reflected back to the observer without the tail adding or subtracting spectral features of its own. This is characteristic of dust; and in the infrared spectrum of some comet tails there is the signature of silicates, the main constituent of ordinary rocks on Earth. For this reason, the Type II tail is called a dust tail, even though we suspect there is an intimate association of sticky, dark organic matter with the fine silicate dust particles.
Type II tails are clearly made of innumerable fine particles. If only Newtonian gravitation were working, a collection of fine particles could not travel through space as if it were a solid body. Instead, each particle would be on a separate orbit about the Sun, a microplanet moving in an almost perfect vacuum. Because the initial speeds of the dust grains leaving the comet depend on the properties and orientation of the puff of gas that carried them off, some will be traveling a little faster and some a little slower than the comet. In their orbits, Mars travels more slowly than the Earth; and the Earth more slowly than Venus. In the same way, faster particles will move outward and slow down, while slower particles will fall inward and speed up. This slow gradient in velocity accounts, both qualitatively and quantitatively, for the characteristic curvature of the Type II comet tail. The great curved yellow tails of comets imply, by their very shapes, individual small particles in separate orbits around the Sun.
Tiny particles—clusters of still smaller motes of silicates and organics—are jetted off the cometary nucleus, and redirected back away from the Sun. Newtonian gravity will then produce the graceful curved tail. But what is this mysterious influence driving the particles back? The first person to guess the right answer was Johannes Kepler, who held that the tail of a comet is pushed away by the pressure of sunlight. The net result, he argued, was that the comet would ultimately dissipate into interplanetary gas.
Schematic representation of the pressure of sunlight blowing back the gases and fine particles in the cometary coma to produce the tail. Diagram by Jon Lomberg/BPS.
Radiation pressure is not a factor in everyday life. Even very small people are not thrown to the ground by sunlight as on a cloudless day they step out of doors. The force of radiation pressure is the equivalent of the weight of a layer one atom thick at the surface of the Earth. Radiation pressure amounts very nearly to nothing. But if you are made of almost nothing in the first place, radiation pressure can push you around. The total force of the sunlight on us depends on how much area we present, but the resistance we offer to sunlight in its effort to push us around depends on our mass. The smaller a particle is, the more area it presents for its mass. Eventually, in free space, a sufficiently small particle will feel the force of radiation pressure driving it outward from the Sun more strongly than the Sun’s gravity pulling it inward toward the Sun.
Turbulence in the tail of Comet Brooks (1893 IV). Courtesy Lick Observatory, University of California.
Both the radiation pressure force and the gravitational force vary inversely as the square of the distance from the Sun; so once a particle is small enough for radiation pressure to dominate, it is continuously accelerated out of the solar system. If radiation pressure wins over gravity at the orbit of Mercury, it is still winning over gravity by the time the particle is halfway to the nearest star. But dust particles must be very small—less than a ten-thousandth of a centimeter across, too small to resolve even with an ordinary microscope—for radiation pressure to eject them from the inner solar system. Therefore, of the dust particles that are blown off the nucleus of a comet by the jets, only the smallest particles are driven back into space by sunlight. The bigger particles—having achieved escape velocity from the comet, but not from the Sun—establish individual orbits about the Sun.
When the particles are much smaller than this, they are even smaller than the wavelength of most of the sunlight, and the particles slip through the crests and troughs of the lightwaves and again are not driven outward. Thus there is only a small range of particles with sizes in the vicinity of the wavelength of yellow light that are accelerated by sunlight out of the solar system. Interplanetary space should be depleted in particles of this size.
If the Type II tail is made of dust, what is the composition of the longer, still more prominent Type I tail? Aristotle had thought that comet tails bore some similarities to the aurora borealis, a view echoed by Kant:
The Earth has in it something which may be compared with the expansion of the vapors of comets and their tails, namely, the Northern Lights, or Aurora Borealis.… The same force of the rays of the Sun, which makes the Aurora Borealis, would produce a vapor head with a tail, if the finest and the most volatile particles were to be found as abundantly on the Earth as on the comets.
While it was not understood in the times of Aristotle or Kant, today we do know something about the aurora—the multicolored time-variable pattern of lights in the sky, sometimes resembling rustling drapery—seen chiefly in the arctic and antarctic regions of the Earth. The aurora appears in the polar regions because it is there that charged particles—especially protons from the Sun—are guided by the Earth’s magnetic field. The protons pour into the atmosphere above the poles, tear the molecules of air apart a little bit, and induce them to glow. T
he aurora is variable because the supply of protons is variable. While the purported similarity in appearance between the aurora and the Type I tails of comets may be less than obvious, it is certainly true that electricity and magnetism are central to the understanding of both.
A prominence of protons, electrons, and helium ions rushes off the Sun, producing a disturbance in the solar corona. Still more energetic eruptions propel blobs of charged particles off the Sun altogether, constituting a solar flare event. Courtesy National Aeronautics and Space Administration.
The spectra of comas mainly show, as we have said, such molecular fragments as C2, C3, and CN. But when the telescope points away from the coma and toward the tail, a completely different spectrum is seen, in which the lines of CO+ are dominant. CO+ is a molecule of carbon monoxide with an electron removed. Such electrically charged molecules are called ions. CO+ preferentially absorbs blue light from the Sun, which, through fluorescence, is re-radiated again in all directions as the same blue light. This is why Type I tails are called ion tails, and why they glow bluely. If you could somehow turn off these particular frequencies of blue sunlight, the dust tail would look almost the same, but the ion tail would be invisible. There is only a relatively small amount of CO+ in the tails of comets, but if you took the CO+ away, the blue ion tail would be replaced by a much dimmer red ion tail, due to H2O+.
Molecules are ordinarily electrically neutral, the number of negatively charged electrons on the outside of the atom just balancing the number of positively charged protons in the inside. Consider a molecule of water sitting in interplanetary space near the Earth. It is struck by a photon of ultraviolet light from the Sun (or a proton), and one of its electrons is carried off into space. The molecule is therefore positively charged: it has more protons than electrons. The charged molecule is called an ion, and the process that carried off its electron is called ionization. If one electron had been stripped off, we would symbolize such a water molecule as H2O+, the plus sign indicating an excess positive electrical charge, or equivalently a deficit of one electron. If two electrons had been carried off, we could write this as H2O++. The same is true for CH+, N2+, or CO+.