However, the paucity of this material is at first puzzling. In the last hundred thousand years or so, all the zodiacal particles now in the inner solar system should have been provided by the dust tails of comets. The solar system is 4.5 billion years old. So why isn’t there much more zodiacal dust? You might very well expect 4.5 billion/100,000=45,000 times more zodiacal dust than there is; in which case, the zodiacal light would be brighter than the planets and the stars. Some biblical fundamentalists have even used this puzzle to argue for a solar system no more than 100,000—and hopefully (in their view) less than 10,000—years old, in a brave but hapless attempt to reconcile a literal reading of the Book of Genesis with the findings of modern science.
The great mass of fine particles released from comets over the eons is missing. Where have they all gone? They have, it turns out, been devoured by the Sun. Small particles around a few tenths of a micron across are lost from comet tails and from the inner solar system altogether, accelerated out into the depths of space by the pressure of sunlight. Such particles seem to have been detected by Pioneers 8 and 9, among other interplanetary spacecraft. Larger particles also feel the radiation pressure, although it is not enough to drive them significantly outward. But it does serve to lessen the Sun’s gravity, making the particle a little lightfooted. There is another, contrary influence, first described by the British physicist J. H. Poynting: The particle is hot, he said, and in consequence, emits radiation to space. Of course, it also reflects sunlight. But because of its motion, it
crowds forward on its own waves emitted in front, and draws away from those emitted behind, so that there is increase of pressure in front and a decrease behind. Thus, there is a force resisting the motion.*
Since now the particle isn’t moving as fast as is necessary to balance the Sun’s gravity, it falls a little inward, where it is still hotter, reflects more sunlight, moves faster, and generates still more resistance to its motion. Slowly the particle spirals into the Sun. Poynting calculated that a tiny particle of rock with a radius of ten microns would reach the Sun in less than a hundred thousand years. Much larger particles are too big to be pushed around by sunlight, and do not spiral in. This fiery fate of small particles orbiting the Sun is now known as the Poynting-Robertson Effect. (The American physicist H. P. Robertson later made the most general formulation of the phenomenon.)
Poynting went on to describe how particles of various sizes will eventually move in orbits
so different that they may not appear to belong to the same system. In the course of time they should all end in the Sun. Perhaps the Zodiacal light is due to the dust of long dead comets … It appears just possible that Saturn’s rings may be cometary matter which the planet has captured, and on which these actions have been at play for so long that the orbits have become circular.
So a typical zodiacal particle spends about a hundred thousand years before it is gobbled up by the Sun, and essentially all the particles in the zodiacal cloud are replenished in such a period. Thus, the great majority of the particles you see in the zodiacal light left their comets of origin long before recorded history; but almost none of them before the human family first evolved. All told, about ten tons of interplanetary particles spiral into the Sun every second, 300 million tons a year.
In its annual voyage around the Sun, the Earth runs into particles of the zodiacal cloud, mainly in the dawn hemisphere; more rarely, fastet moving debris catches up with the Earth in its twilight hemisphere. The total being accumulated, all over our planet, is about a thousand tons of dust a day. The number of fine particles collected by stratospheric aircraft—U-2’s and others—is about what we would expect for particles in the zodiacal cloud captured by the moving Earth. If this cometary dust had fallen on the Earth over its entire history at the same rate that it does today, and if nothing destroyed it after landfall, there would be a dark, powdery layer about a meter thick everywhere on Earth. (If a single large comet were pulverized, and all its debris were spread smoothly over the Earth, a layer about a centimeter thick would result.)
The zodiacal light as seen in Europe by a M. Heis, from Amédée Guillemin, The Heavens (Paris, 1868).
The View from the Golf Course
Even the impact of a smallish fragment of a comet (or asteroid) with the Earth can have significant effects far away. The day after the Tunguska impact in Siberia, on June 30, 1908, the following letter was written to the Times of London. It was published two days later:
Sir,—Struck with the unusual brightness of the heavens, the band of golfers staying here strolled towards the links at 11 o’clock last evening in order that they might obtain an uninterrupted view of the phenomenon. Looking northwards across the sea they found that the sky had the appearance of a dying sunset of exquisite beauty. This not only lasted but actually grew both in extent and intensity till 2.30 this morning, when driving clouds from the East obliterated the gorgeous colouring. I myself was aroused from sleep at 1.15, and so strong was the light at this hour that I could read a book by it in my chamber quite comfortably. At 1.45 the whole sky, N. and N.E., was a delicate salmon pink, and the birds began their matutinal song. No doubt others will have noticed this phenomenon, but as Brancaster holds an almost unique position in facing north to the sea, we who are staying here had the best possible view of it.
—Yours faithfully,
Holcombe Ingleby
Dormy House Club, Brancaster, July 1.
These tiny grains have had an epic history. Intermingled with specks of ice, they were for eons floating in the gas between the stars; then caught up in a contracting, spinning interstellar whirlpool that eventually formed the solar system; growing to form cometary nuclei that were promptly ejected into cold storage at the outskirts of the solar system; then, plunging toward the Sun, propelled out from the comet as the ices evaporate; orbiting the Sun as individual microplanets; and, some of them, finally spiraling in until—during their passage through the solar corona—they are turned into puffs of gas.
The constituent atoms—silicon, oxygen, iron, aluminum, carbon, hydrogen, and the rest—are then diffused through the upper atmosphere of the Sun, eventually being gathered into its internal circulation, and carried down deep into its interior. Some of these atoms will be transported to the very core of the Sun, and engage in the thermonuclear alchemy that makes our star shine. But they constitute the most minor of contributions. Every now and then, though, an occasional light beam, an odd photon—perhaps illuminating for an instant the way of a gnat—originates in the belt of distant comets.
You take even a casual glance at most of the worlds in the solar system, and you discover that they’re full of holes. There aren’t many analogues that come readily to mind for this sort of battered surface—a fact that says something about the Earth. People once compared the cratered surface of the Moon to Swiss or Emmenthaler cheese, but that doesn’t quite evoke its real appearance—craters upon craters upon craters, down to the smallest crater you can see. Some of these worlds look more like a rain-spattered beach. Drop marbles or BB’s of different sizes at random in plaster of paris, and let the surface set. The craters will be nicely circular, with rims and ramparts, and occasionally a central mountain peak. Some craters will overlap others. The plaster of paris begins to look very much like the surface of a world.
Some of these impact craters on the Moon, say, are made by asteroids, but most—especially in the outer solar system—are made by comets. Rarely, a world will overtake a comet. More often the comet will overtake the world, or crash into it head-on. Like the Sun, the moons and planets collect their share of cometary impacts. The Sun, being made of gas, can retain no impact craters. But worlds with uneroded ancient surfaces remind us at a glance of how many comets have, over the eons, died in fatal collisions.
On June 30, 1908, something fell out of the sky in Siberia and, at an altitude of eight kilometers, exploded and knocked down a forest. The blast was more powerful than that of the highest yield nuclear weapon
in the current arsenals. No impact crater was ever found. The responsible agent was once thought to be a good-sized but still fragile piece of Comet Encke. It is now known that the Tunguska explosion was caused not by a comet and not by a metallic asteroid, but by an ordinary rocky asteroid exploding in the middle air but with an energy of about 1 megaton equivalent of TNT, the equivalent of a million tons of high explosives.
Comets and asteroids that fall apart before reaching the surface leave little trace. But beyond that, objects that actually strike the surface at high speed leave unmistakable signatures of their past presence. Because the falling body is traveling very fast, the hole it excavates will be larger than the falling body. If no subsequent geological event obscures the surface, every crater corresponds to an impact, and almost every impact corresponds to a crater. The surface of a moon or planet is therefore a diary of assault and battery. If you know how to decipher that record, you can uncover the catastrophes of ages past.
Saturation cratering on the surface of Saturn’s moon Rhea. Most of these large holes in the ground have been made, over billions of years, by cometary impact. Voyager 1 photograph courtesy National Aeronautics and Space Administration.
Take, for example, the Earth’s Moon. The side that permanently faces the Earth and that we know with the naked eye has two kinds of terrain—dark, smooth lowlands and bright, rough highlands. Both highlands and lowlands are cratered, but there are many more craters in the highlands than in the lowlands. Because American astronauts and Russian robots have brought back samples from nine different locales on the Moon, we know something of the composition and—through radioactive dating—the ages of various provinces of the lunar nearside.
The dark lowlands are made of lava that, 3.3 to 3.9 billion years ago, gushed out of the then-hot lunar interior, wiping out any previous craters. Thus, for the lowlands, the earliest master record has simply been erased. The sparse number of lowland craters is just what we might expect if the inner solar system had always been as full of comets and asteroids as it is today. But the highlands have far too many craters for that. Here craters have been superimposed on preexisting craters—so that again a record of the earliest times has been lost. One way or another, these worlds hide the evidence of their very beginnings, although the record of later catastrophes is clear.
From the story told by the dense cratering in the lunar highlands, it is clear that, in the first few hundred million years of lunar history, there were enormously more impacting objects around—comets and asteroids—than there are in interplanetary space today. The same story is told on world after world as our reconnaissance spacecraft fly out among the planets. On Saturn’s moon Rhea (see this page), for example, there has been no geological activity for billions of years, and we are presented with a world cratered to saturation, from pole to pole. In the beginning, interplanetary space must have been full of boulders and icebergs that crashed into the forming worlds. Indeed, this is how the worlds formed, in violent collisions.
Not all the worlds retain the craters of eons past. On some, as in the lunar lowlands, the traces have been eradicated. Something fills the craters in, or rubs them out, or covers them over. On Venus, there are recent lava flows; on Mars, great sand drifts; on Jupiter’s moon Io, a surface rich in recently frozen sulfur; and on Enceladus, a satellite of Saturn made almost entirely of ice, something has melted the surface—the cratering record on that world was literally writ on water. On Io, the craters may be wiped out in centuries; on Venus, it may take half a billion years. But stretching through the entire solar system is, written on top of one another, the chronicles of ancient impacts and, fairly often, of more recent geological processes.
Something similar is true on Earth, where even in arid terrain running water rather quickly destroys the craters—so that unless they are very recent, they are very hard to find. So-called Meteor Crater Arizona is only about 25,000 years old (see below). Most terrestrial impact craters are much older, and these are generally large or in geologically inactive regions.
Sometimes a collision scar will reveal something of the underlying planetary surface. On Mars, for example, there are craters with scalloped flow patterns around them, indicating the presence of subsurface water ice that has been momentarily liquefied in the collision; surface debris is carried outward until the water freezes. The impact of a large comet might bring water or an atmosphere to worlds that are nearly airless.
Meteor Crater, Arizona. This crater is 1.2 kilometers in diameter and was probably produced 15,000 to 40,000 years ago when a lump of iron 25 meters across impacted the Earth at a speed of 15 kilometers per second. The energy released was roughly equivalent to that of a 4-megaton nuclear explosion. It is, of course, a meteorite crater, not a meteor crater. Courtesy of Meteor Crater Enterprises, Inc.
When a comet strikes a gas planet like Jupiter, it sweeps through the upper atmosphere, encounters more and more resistance as it plunges deeper, and somewhere below the visible clouds breaks up, its material eventually circulating over much of the planet. The stuff of comets is mixed with the air of Jupiter.
Craters have shapes. Some are perfectly bowl-shaped; others are flat, shallow, gently sloped. The shape of the crater does not much depend on the velocity of the impacting object, provided it hits the ground hard enough. The result is the same as detonating a large explosion at the point of impact. The crater shape does depend on how soft the surface is, and on the fragility of the infalling material.
American astronauts on the lunar surface photographed craters too small to see with the unaided eye. Samples returned to Earth have also revealed a plethora of microcraters. Some must be due to asteroidal dust swept up by the Moon; others to fine particles generated on and sprayed over the Moon by large impacts. We also expect some fraction of the lunar microcraters to arise from cometary dust, excavated puff by puff. The bowl-shaped microcraters seem to be due to cratering by fine, rocky particles—from asteroids or cometary silicates. But there is also a population of microcraters that are extremely shallow for their size, more depressions than bowls. The shallow microcraters can only be understood if they were excavated by low density grains impacting the Moon, and these can only be cometary in origin. Microscopic bits of cometary fluff are very gently softening and abrading the face of the Man in the Moon.
And by collision roulette, larger comets must also hit the Moon from time to time. A lunar soil sample called 61221 shows evidence of the molecules H2O, CH4, CO2, CO, HCN, H2, and N2. Volatiles present in this sample—especially HCN—and absent in other lunar samples are suggestive of a recent cometary impact. There is now plentiful evidence of abundant organic matter in comets. In a collision, comets can also impress a pattern of magnetism around the impact area, and this may account for some otherwise puzzling magnetic anomalies found on the Moon.
Sooner or later, comets in the inner solar system run out of gas. Typically, the amount of water ice that disappears in a single perihelion passage is a few meters (about 10 feet)—as we’ve already mentioned. Each time the comet passes by the Sun, it shrinks. Of course, the comet is not pure ice, but an intimate mix of ice and dust. Huge quantities of dust are blown off in the great jet fountains that sometimes play within the coma, and dust is also lifted more sedately by evaporating ice. So a comet made mainly of ice will lose ice and dust every perihelion until the comet is altogether gone. Nothing is left but a powdery contribution to the zodiacal cloud and, perhaps, an occasional sporadic meteor in the skies of Earth.
But now imagine that the comet has more dust than ice. After the first perihelion passage, there is a layer of dust left on the surface that geologists would call a lag deposit. Some of the dust has been carried away by the evaporating ice, but not all. The next time the comet approaches the Sun, the dust serves as insulation for the underlying icy dirt. It is now harder for it to heat up. And if it does heat up, it is harder for the underlying vapor to escape—the way is barred by the layer of dust. The pent-up pressures of subsurface gases
may blow off the overlying mantle of dust, and then the process begins again. After a number of perihelion passages, so deep a lag deposit may be created that no further ice can be lost to space. No longer does the comet form a coma or a tail. It shuts down, closes up shop, becoming a small, dark lump of matter in the inner solar system.
Something similar will occur if an icy comet with a rocky core periodically enters the inner solar system. Such a comet would have to be very large—at least tens of kilometers across and perhaps hundreds—for it to have melted and vertically segregated materials of different properties, the denser rock on the inside, the lighter ices on the outside. Thus, there are at least two ways for comets to evolve into small bodies with surfaces of silicates and organics. But there is a category of such small, dark objects, some of them in quite eccentric orbits, called the Earth-approaching asteroids. The entrancing suggestion that comets might make a metamorphosis from an icy world to a rocky one was first made by Ernst Öpik (Chapter 11).