The chief problem is the cold. Comets in the Oort Cloud are so far away that the feeble sunlight is hardly able to warm them at all. They stay far below the freezing point of water or other abundant liquids. But with no liquids (and no atmospheres), it is hard for life to get going: the molecules are stolid, virtually immobile. The elaborate chemical processing that must have preceded the origin of life on Earth is unlikely to occur at 10° above absolute zero. Even if all the required organic building blocks were present, at these temperatures they simply could not interact sufficiently. But sunlight is not the only source of heat.
The interior of the Earth is warm, in part because of a smattering of radioactive elements—uranium, thorium, potassium. Every time one of these elements decays, a gamma ray or a charged particle is ejected which then strikes neighboring molecules, mainly silicates, and heats them up. This is, in part, why it is hotter at the bottom of a deep mine shaft than it is on the surface.
Now, as radioactive atoms decay, they fall to pieces with a statistical regularity. There is a characteristic time, called the half-life, in which half the atoms of a given radioactive species have decayed. The half-lives of uranium, thorium, and potassium are hundreds of millions to billions of years—comparable to the age of the Earth. This is no coincidence: All the radioactive atoms with shorter half-lives have already decayed. They may well have been here once, but they are gone now, decayed into other, more stable kinds of atoms. So in the early history of the solar system, there must have been considerably more radioactive heating, and it is more likely that comets were internally warmed as the solar system started out.
The bigger the cometary nucleus, the longer it takes for heat to be conducted to the surface and for the interior to cool. Thus, if early heating had been provided by a now extinct radioactive atom called aluminum 26, the core of a comet twenty kilometers across could have been kept liquid for ten million years; and a two-hundred-kilometer comet for a billion years. So something like the Great Comet of 1729 might just conceivably have been carrying an ocean inside. A comet with a hundred-kilometer radius, made mostly of water and other liquids, would be enough to cover the entire surface of the Earth to a depth of ten meters. An interior ocean of such dimensions, rich in organic matter and given four billion years for subsequent chemical interaction, is an interesting sort of place. Is it likely that life could develop there?
Comets cannot be homes of life; they are not sufficiently condensed; indeed, they are probably but loose congeries of small stones. But even if comets were of planetary size it is clear that life could not be supported on them; water could not remain in the liquid state on a world that rushed from one such extreme of temperature to another.
—E. WALTER MAUNDER, ARE THE PLANETS INHABITED? LONDON, 1913
It is much easier to imagine the origin of life on the surface of the early Earth, where there was a planet-sized ocean and abundant sunlight. Let us, therefore, put aside the idea that life now or in the beginning fell to Earth with the comets, and ask about the origin of life from organic molecules on Earth. Life on our planet is built entirely around a handful of molecular types, the most important of which are the nucleic acids and the proteins. If we could understand the large-scale production of those molecules on the early Earth, we would have made significant progress toward understanding the origin of life. The conventional scientific wisdom today—once wildly controversial—is that the key molecular building blocks were spontaneously formed on the early Earth in obedience to well-established laws of physics and chemistry. Molecules in the hydrogen-rich primitive atmosphere were broken apart by ultraviolet light from the Sun, lightning, or even the shock waves produced when meteors enter the atmosphere at high speeds. The fragments are known spontaneously to recombine (see the diagram on this page) to form the stuff of life.* It happens in the laboratory, and it must have happened on the early Earth. Ironically, cyanides, the deadly poisons that produced the great comet scare of 1910 (Chapter 6), seem to be an essential intermediary in the origin of life (see this page). So could comets have contributed to the origin of life—not by carrying life to the Earth, but by conveying the building blocks out of which life arose?
We have seen that in the early history of the Earth—before the inner solar system was swept clean by collision and ejection—comets were much more common; an unmistakable record of these times is preserved in the impact scars on the moons and planets. In the first few hundred million years after the formation of the Earth, the flux of comets was much greater than in the more sedate present. It appears that there were at least thousands of times more comets then, and perhaps still more. These comets, made mainly of frozen water, would, on impacting the primitive Earth, of course have carried water with them. Over the first few hundred million years of Earth history, they would have deposited some 3 × 1015 (or 3,000,000,000,000,000) tons of water. But this is enough to cover the whole surface of the planet to a depth of almost six meters (20 feet). If the number of comets in the inner solar system was larger, then the Earth would have accumulated still more water. Just possibly, most of the water in the oceans arrived via comet special delivery after the Earth was fully formed.
Certainly, comets were not the only source of water. The clays in carbonaceous asteroids contain up to 20 percent water. Molten lava arriving at the Earth’s surface from the interior is known to carry with it a few percent of water, and volcanic events were frequent billions of years ago. Even so, it is striking that comets could have done so much to fill the early ocean basins with water.
The waters that the comets brought were flavored with perhaps twenty-five percent of complex organic matter. So did comets bring the ingredients for the origin of life? This is not so different from the view of the Jukun people of Nigeria, who hold that meteors represent a gift of food from one star to another. Everything depends upon the timing. If the early comet flux fell off while the Earth’s surface was still molten, then no matter how complex the organic matter in the comets may be, it would all have been fried in the magma; the origin of life would then have had to occur later, and without a significant contribution from the comets.
If the early Earth were composed of gases such as hydrogen, methane, ammonia, and water, amply supplied by energy sources including ultraviolet light from the Sun, then the amount of complex organic matter available for the origin of life becomes quite plentiful; and cometary sources of organic matter to supply the origin of life seem at least in part superfluous.
But looking at the geological record, most geologists conclude that the atmosphere of the early Earth was largely composed of carbon dioxide. Irradiating mixtures of carbon dioxide and other gases yields a paltry quantity of organic matter. In that case we are forced upon the alternative of a cometary supply. Because the cratering rate throughout the solar system was so much larger four billion years ago than it is today, it is a safe guess that the supply of organic matter-carrying comets was also hugely greater. However, a large comet impacting the Earth at high speed becomes so hot that almost all the organic matter it contains becomes fried, vaporized, and not at least immediately usable. On the other hand, interplanetary dust particles of cometary origin float through the early atmosphere down to the surface with almost no attrition. While they do not provide as much organic matter as a hydrogen-rich primitive atmosphere, they may provide enough. In addition to the simplest hydrocarbons, methane, ethane, methyl alcohol, hydrogen cyanide, and formaldehyde—all very useful for the origin of life—have been detected. The elemental composition of comets as is now known plus the detailed chemical analysis performed on Comet Halley by Soviet and European spacecraft after its 1986 apparition are remarkable in the abundance and variety of organic matter detected. Even if most of the primordial organic matter on Earth was indigenous—generated in the air, water, and land of our planet—it is conceivable that some critical chemical components originally arose on comets.
Layered deposits near the South Pole of Mars, signs of a complex and episodic (perhaps periodic) geolo
gy. The picture is almost 200 kilometers across. One day there will be machines and people examining these sediments to better understand the past history of Mars. Viking Orbiter photograph. Courtesy National Aeronautics and Space Administration.
An ancient river valley snakes across the battered and cratered martian landscape. Viking Orbiter photograph. Courtesy National Aeronautics and Space Administration.
Where does the organic matter in comets come from? The bulk of it, almost certainly, is of interstellar origin. Grains of dust around stars and in the vast, nearly empty darkness between the stars collide and are irradiated and, molecule by molecule, a kind of cosmic transmutation takes place over eons. The grains are collected into comets (and planets and humans) with the complex chemistry that may reflect different regions of the Galaxy and different epochs. It is a stirring thought that events thousands of light-years away and billions of years ago may have contributed not just to our origins, but to the very organic chemistry that makes us up.
There is nothing in this argument unique to the Earth. Other nearby worlds in our solar system and beyond, in their early histories, must also have been pummeled by the comets. Oceans must have accumulated on Venus and Mars too, even in the unlikely circumstance that no water from their interiors reached the surfaces of these planets. Indeed, for both Venus and Mars there is some evidence of ancient oceans.
On Venus, a world now depleted of water, the little hydrogen still left is swiftly escaping to space. Lightweight molecules escape more readily than heavier ones, because they are more likely to be induced, by random collision, to move in excess of the escape velocity. The heavy form of hydrogen, deuterium, escapes more slowly than the more common, lighter variety. As time goes on, the amount of deuterium relative to hydrogen on Venus should increase. Thus, from the present proportions of hydrogen to deuterium, it is possible to estimate how much water was once present. In this way Thomas Donahue of the University of Michigan and his colleagues have deduced an ancient ocean on Venus, now lost—a conclusion not accepted by everyone.
Today Venus is a desolate world, its surface temperature raised to a broiling 480°C (roughly 900°F) by a huge atmospheric greenhouse effect dominated by carbon dioxide. But this CO2 was not outgassed into the atmosphere overnight, and it is possible to imagine a much more clement early environment, with oceans covering the surface, and dissolved cometary organic molecules colliding, interacting, growing still more complex. The turning off of the high early flux of comets and the outgassing of carbon dioxide from the interior may have changed Venus from a tropical paradise to an inferno. An important question for future exploration remains: Is there any trace today of the ancient oceans? Is it possible that life arose on Venus billions of years ago, with some durable remnant or fossil awaiting future explorers?
These questions can be asked with still greater seriousness for Mars, because there a range of evidence exists for abundant water today—frozen in the polar caps, buried subsurface, and chemically combined with the soil. Although liquid water is absent from Mars now, the data suggest that water ran in rivers and floodplains a billion years ago. It is even possible that evidence of shorelines and other signatures of ancient martian oceans exists in images from the Viking orbiters. Mars is a smaller world than Venus or Earth, so a major fraction of its early atmosphere could have since escaped to space. It is also a colder world, so much of the surviving cometary water may now be locked away as ice.
The worlds in this part of the solar system show the tokens and remnants of the great flux of comets that filled the inner solar system in those long-gone first times. We can think of the present modest infall of cometary debris as a feeble reminder of an epoch that lasted for hundreds of millions of years, and changed the face of the solar system.
If something close to the present oceans were carried as comets to the primitive Earth, simple calculations show that they would also have brought an amount of carbon comparable to that in the entire sedimentary column of the Earth—in all the carbon in the rocks for kilometers down, all the living things, all the humus in the soils, all the petroleum, coal, peat, graphite, and diamonds, all over the Earth. If anything like this is true, then it is fair to say that the surfaces of the Earth and the other terrestrial planets, after they were almost fully formed, were dusted with a layer of cometary stuff a few kilometers thick. This cometary coating is, relatively speaking, thinner than the confectioner’s sugar on a jelly doughnut. But it means the world to us: On this planet, at least, the comet dust has come to life.
The early Earth was covered by impact craters, large and small; and still greater excavations called basins. Most of the water carried by the comets was vaporized on impact, and fell out as rain. So comets dig big holes, in effect fill them with water, and salt them with complex organic matter—performing these duties mainly in the earliest history of the planets when the maximum time is available for life to arise. Again, nothing in this recounting is restricted to the Earth; the same thing may have happened on countless other worlds in the Milky Way. It is hard not to think of the comets as cosmic elves, dashing through space and conferring the potential for life on the worlds of this solar system and countless others.
The Vega 2 multinational spacecraft, organized by the Soviet Union. Illustration courtesy European Space Agency.
So take a look around you. If that comet flux after the surface of our planet cooled was many orders of magnitude more than it is now, what comes from Earth, and what from comets? Everything alive would derive ultimately from comets—all the plants and animals and microbes and men and women. All the buildings, railroads, highways, sculpted farmland, songs, submarines, space vehicles—these are all made by humans, and for this reason alone are derived from comets. Even the daytime sky comes from the comets, because O2 and N2 are made by life. Well, at least the rocks are from the Earth, we might argue, at least the mountains. Yes, but the rocks are oxidized, chemically altered and broken down by water and by life. The mountains are sculpted and eroded by water. When a lump of the underlying planet tries with exquisite slowness to poke up through the comet stuff, it is ruthlessly worn down. Even in the wasteland of Antarctica, the landscape appears to be, one way or another, comet-derived. It seems possible that the only things we see that exist entirely apart from comets are the Sun and the stars. A layer of comet stuff once powdered this world, and during the subsequent four and a half billion years the dust has made a little progress. It has developed complex beings, aspirations, a first try at intelligence. At last it is turning its attention to the comets from which it came.
The multinational Giotto spacecraft organized by the European Space Agency. Courtesy ESA.
*The ablation spheres are also found in dust samples collected at stratospheric altitudes, but they are dominated by the smaller, more fragile particles which were able to cool rapidly on entry and avoid melting. Fewer of the spheres fall on the surface; but they are comparatively hardy. Down on the ground, it’s much easier to find ablation spheres than organic flakes.
*Simple nucleic acids in the right environment can make other nucleic acids out of smaller molecules. Simple proteins can regulate the chemical reactions in their neighborhood. Thus, while forming nucleic acids and proteins falls short of making life from scratch, it is a major step in that direction.
PART III
Comets and the Future
CHAPTER 18
A Flotilla Rising
By the middle of the Twentieth Century, it will have rounded its remotest stake … and will once more have begun the long journey sunward, which will not end ’til 1986. Then again will telescope and camera, spectroscope and photometer, be pointed Halley-ward, just as eagerly as today.
—DAVID TODD, HALLEY’S COMET, AMERICAN BOOK COMPANY, CINCINNATI, 1910
There was a time—and very recently—when the idea of learning the composition of the celestial bodies was considered senseless even by prominent scientists and thinkers. That time has now passed. The idea of a closer, dire
ct study of the universe will today, I believe, appear still wilder. To step out onto the soil of asteroids, to lift with your hand a stone on the moon, to set up moving stations in ethereal space, and establish living rings around the Earth, the moon, the Sun, to observe Mars from a distance of several tens of versts, to land on its satellites and even on the surface of Mars—what could be more extravagant! However, it is only with the advent of reaction vehicles [rockets] that a new and great era in astronomy will begin, the epoch of a careful study of the sky.
—KONSTANTIN TSIOLKOVSKY, INVESTIGATION OF
WORLD SPACES BY REACTION VEHICLES, MOSCOW, 1911
Shortly before dawn, a rifle shot rang out and a moment later the whiz of the bullet could be heard, narrowly missing the three occupants of the gondola. Despite the fact that it was May, they were heavily bundled up. In the early hours of the morning it gets very cold a kilometer or two above the Earth. But they were only 500 meters high when they were fired at. It must have been an unusual sight—the great balloon and its suspended gondola illuminated by the rising Sun when it was still dark in the Connecticut countryside below. Perhaps the balloon had startled some farmer who felt his only recourse was to shoot it down, whatever it was. Or perhaps the bullet was merely a safe protest against the idle rich and their expensive diversions.
But this was not a pleasure cruise. Aboard was Dr. David Todd, professor of astronomy and navigation and director of the observatory at Amherst College. Todd’s specialty was the astronomical excursion. During his professional career, he led expeditions to the Dutch East Indies, South America, the Barbary Coast, Russia, Japan, and West Africa—in each case to observe an eclipse of the Sun. He once dismantled the college’s observatory and crated the telescope to the Andes, in part to look for signs of life on Mars. Now, in May 1910, Comet Halley was in the skies, and David Todd had risen to the occasion—not very far, it is true, but high enough to get above some of the distortion caused by the Earth’s atmosphere. He had taken aboard a small two-and-a-half-inch telescope, magnification 30 times, with which he hoped to observe the comet. The night was clear and the vibrations from the balloon minor.