(Appeared in an anthology: “God, the universe and men – Why do we exist?”
(ed. Wabbel, T.D.), Patmos, Dusseldorf, 2003 (original in German).
A Neolithic comet
Comets are jokers in the celestial pack. They irrupt, usually without forewarning, into the orderly progression of the sky. They cross the celestial sphere in weeks or months, growing one or more tails, before fading and disappearing from sight. On rare occasions a comet may be an awesome sight, and the historical literature of the past two thousand years is sprinkled with accounts of the fear induced when a great comet, its smoky red tail bisecting the heavens, appears in the night sky. In the remote past, tales of such apparitions were often conflated with stories of disaster on Earth. A comet called Typhon in Greek mythology was connected with a mythological flood, and the legend of Phaethon, in which the sun’s chariot went off course and the Earth was first burned up and then flooded, may describe an exceptional meteorite impact. There is good evidence that the sky in Neolithic times was dominated by a recurrent, giant comet, and that the Earth annually ran through an associated meteor storm of huge intensity. The origin of religion dates to these times and may be tied up with this spectacular night sky. The prospect that cosmic myths, megaliths and art dating from this time may have been responses on the ground to threats in the sky has in recent years moved from Velikovskian fancy to a subject for serious scholarly discussion. In more scientific times, too, it was often suggested that a comet striking the Earth might create create worldwide havoc. For example past encounters of Halley’s comet were supposed to have coincided with Noah’s flood in 2342 BC. This catastrophist view of Earth history was widely held until late 1830s. From about the middle of the 19th century, however, it was supplanted by a uniformitarian one, at least in the English-speaking world. Partly this came about because geologists came to recognise that the terrestrial landscape had been formed over aeons by gradual forces, by erosion and slow mountain building. The astronomers too played their part in this changing perception. Several periodically returning comets were found to be associated with annual meteor streams. It seemed that the end state of comets was nothing more exciting than a swarm of dust. By the time of the Victorians, the universe was seen as a more or less irrelevant backdrop to the affairs of Earth. Scientists were free to explain evolution unhindered by any thoughts of celestial disturbance. The occasional revivals of the catastrophist worldview became the domain of cranks. This long slumber lasted until the late 1970s.
The destroyer of worlds
Starting in the late 1970s, however, there came a renaissance of catastrophism. A number of factors brought about this change of perception. First, wide-angle telescopic surveys of the sky were revealing the existence of a population of small, fast-moving Earth-crossing asteroids, making impacts on Earth quite common events on geological timescales. Second, large impact craters were being found on Earth in increasing numbers. Usually these were overlain with sediment or had otherwise gone unrecognised, being seen by Earth-minded geologists as no more than, say, circular lakes. An example is Lake Manicougan in Canada, 80 km across, about the size of the Copernicus impact crater on the moon. The impact rates from both the evidence on the ground and in the sky converged to the same picture: the Earth is a heavily bombarded planet. From this new evidence it was not a large step to infer that the great mass extinctions which populate the fossil record may have been caused by celestial forces. After all, a 10 km comet or asteroid hitting the Earth at 25 km per second would unleash 100 million megatons of energy, equivalent to a small hydrogen bomb on each square km of the Earth’s surface. It is hard to believe that such an event would leave no geological or biological trace! One killing mechanism might be the ejection of dust, cutting out sunlight, decreasing photosynthesis and so collapsing the food chain at its base. The heat generated by a large impact, spread globally by incinerating debris, is another possible killing mechanism: the world would be subjected to a rain of fire. It is not surprising that many people (Urey in 1975, Napier & Clube in 1979, Alvarez in 1980), have over the years proposed that huge impacts are connected with mass extinctions of life such as the dinosaur extinctions of 65 million years ago.
Contrary to the popular literature, however, it is unlikely that the 10 km bolides which generate the biggest craters on Earth are asteroids. Asteroids in the belt between Mars and Jupiter are confined to highly stable orbits. To be thrown out of the belt, an asteroid must be nudged by collision from its stable orbit into a nearby, unstable one.
This asteroid-nudging mechanism probably works for kilometre-sized objects but a big asteroid is just too difficult to shift. The major killers, then — the global destroyers — are likely to be comets or their debris. But as soon as we look to comets as the agents of mass extinction, we enter a whole new ballpark. And overwhelmingly, the most massive objects to enter the near-Earth environment on geologically interesting timescales are rare, giant comets (say 100 — 200 km or more across).
It is very unlikely that such a monster has struck the Earth any time within the past billion years, if only because it would have sterilised the planet. The fate of a short-period comet approaching the sun is to disintegrate. The dust generated by a very large comet will form a temporary disc around the sun, within which the Earth will orbit. We are immersed in such a disc, the zodiacal cloud, now. However in the wake of disintegration of a giant comet the mass of this zodiacal cloud will increase to hundreds or thousands of times its present mass. Immersed in it will be larger debris, ranging from cm sized icy pebbles to multi-km fragments of the original comet. The result is that for a period of tens of thousands of years following the arrival of a supercomet, the Earth environment becomes extremely hazardous. Small dust particles may trickle down for the active lifetime of rare, giant comets — which could be millennia. The resultant cooling may lower the ocean levels by some hundreds of metres, changing the spin rate of the Earth and inducing stresses at the core-mantle interface comparable to those involved in plate tectonic movement. Rapid climatic variations, coupled probably with worldwide tectonic disturbances, are then expected. A cosmic mass extinction is, then, a much more complicated affair than a single huge impact. Rather, it is an affair of multiple impacts coupled perhaps with extensive vulcanisms, lava outpourings and rapid climatic changes.
As it happens, this more complex picture gives a much better fit to the detailed record of events at the well studied extinction boundaries, in particular the stepwise extinctions and severe coolings of the Earth 36 — 39 million years ago, and the dinosaur extinctions of 65 million years ago, which were preceded by a rapid greenhouse warming over 200,000 years followed by a cooling during the last 300,000 years, accompanied by a massive outpouring of lava in India over the same period and the deposition of several layers of small glass spherules in Mexico, the Indian Ocean and other places, indicating, probably, a multiplicity of impacts. Extinctions of many life forms such as bivalves, ammonites and rudists were well under way long before the big impact which gave the 170 km Chicxulub crater of 65 million years ago.
The Galactic connection
The arrival of comets, too, gives structure to the terrestrial record of catastrophes. Many comets derive from the Oort cloud, a great swarm comprising maybe 100 billion comets or orbiting at distances out to 50,000 astronomical units, a quarter of the way to the nearest star. Comets orbiting beyond this distance would be thrown into interstellar space by the gravitational action of passing stars and nebulae. The sun has a tenuous gravitational hold on the Oort cloud comets, and it is not surprising that the Oort cloud is sensitive to the galactic environment. It is is disturbed from time to time, by passing stars, by massive nebulae or when the solar system penetrates the spiral arms of the Galaxy — for our Galaxy is an open-armed spiral. Any such disturbance sends a swarm of comets plunging into the inner planetary system, and with luck we might be able to see the effects of spiral arm penetrations or passing nebulae in the geological record.
In fact, when we examine the age distribution of the well-dated terrestrial impact craters, of which there are less than fifty, a remarkable pattern emerges. There’s a distinct bunching of ages, with clearly identifiable impact episodes (seven in the last 250 million years). Most of the larger craters belong to these impact episodes. There seems to be a background ‘drizzle’ of small impacts — probably strays from the asteroid belt — superimposed on which are powerful bombardment episodes each comprising a swarm of missiles, including major impactors yielding collisions 50 — 100 million megatons in energy. Bombardment episodes of this sort may be expected to yield mass extinctions of life and corresponding geological disturbances and indeed a correlation, albeit a very imperfect one, does exist.
Remarkably, the two greatest mass extinctions coincide with passages of the solar system through the spiral arms of the Galaxy. The closest life came to extinction in post-Cambrian times seems to have been the Permo-Triassic event of 245 million years ago, when perhaps 95% of all species became extinct. At that time the solar system was passing through a spiral arm known as the Scutum arm. And about 60–70 million years ago, covering the epoch of the Cretaceous-Tertiary mass extinction, the solar system was passing through the Sagittarius arm. Without this passage, the dinosaurs might still be with us and homo sapiens would probably not exist: we are children of the Galaxy.
And yet, life has survived, and seems to have existed on Earth, no doubt surviving many impacts and other cosmic disturbances, for a very long time. The evidence for Precambrian life is strong, and was reported as long ago as 1858, not long after the publication of The Origin of Species. The strongest evidence for this comes from stromatolite domes — precipitates from the metabolic activities of sea-floor microorganisms — which have been found in rocks 3,500 million years old in Western Australia.
Earlier than that, we enter the era of the so-called Late Heavy Bombardment. This was a time, comprising the first 10% of Earth history, when the impact rate was perhaps a thousand times higher than it is at the present time: the heavily cratered lunar highlands belong to this era. The heat generated from large impacts, in those days, would have repeatedly raised the temperature of the crust to above 100 degrees Centigrade and so sterilised the Earth. Probably, oceans could not exist because they would simply boil off. Whatever the cause of this tremendous bombardment, it seems to have come to an abrupt end about 3.8 billion years ago. It is striking that life is found on the Earth just as soon as this bombardment subsided.
Without Jupiter and Saturn, comets flooding in from the Oort cloud would subject the Earth to a sustained bombardment comparable to that of the early Earth. But these giant planets act as powerful shields, gravitationally deflecting the long-period comets away from the inner planetary system. We owe our existence to Jupiter and Saturn. This does raise an interesting constraint on the existence of carbon-based lifeforms in other star systems: a sheltered environment is essential. The argument is quite general and depends only on the kinetic energies and impact frequencies of small bodies which, presumably, are a universal accompaniment to planetary systems. We do not know enough about other planetary systems to say how common such environments are likely to be. It is one more ingredient to be thrown into the anthropic pot.
The evidence of the terrestrial record, then, is that life has an ability to withstand cosmic slings and arrows throughout aeons, provided these impacts are not so energetic that they sterilise the planet. This must come down to life’s powers of replication. A single microorganism with a doubling time of one hour, surviving a great cosmic disaster, would replenish a nutrient-rich ocean within a year.
A mechanism for interstellar panspermia
This power to replicate has led to speculation that, perhaps, life is a cosmic rather than simply a terrestrial phenomenon, drifting throughout the Galaxy and taking root where it can. If this were so, it would profoundly affected our perceptions of life’s cosmic significance. But we need a mechanism for interstellar panspermia. And here, unexpectedly, giant comets seem to provide it.
It has long been clear that a large impact may eject materials from a planet. The streaks from the crater Tycho, easily seen with binoculars, cover the entire lunar hemisphere and are evidence that material can be thrown thousands of kilometres from an impact site. A large, fast impact on land on the Earth will throw a mass of terrestrial rock equal to a fraction of a percent of the impactor mass into orbit around the sun. Most of this material will be quickly recaptured by the Earth, some will eventually fall into the sun, but a small proportion, subject to the bagatelle of random planetary perturbations, will eventually be ejected from the solar system altogether. It is estimated that something like 10 tonnes of surface material from the Earth, from past terrestrial impacts, drifts into interstellar space each year. A gram of poor, rocky soil may contain 10 million microbes, and microorganisms will occupy every micro-crack in a boulder. Shielded from ultraviolet light, it is likely that microorganisms could survive for, say, a hundred thousand years before cosmic rays eventually destroy them. This figure may be conservative: survival times of 100 million years against cosmic rays have been proposed.
Here, however, we have a problem. In the form of a handful of boulders, such ejecta would simply lose themselves in interstellar space. It is unlikely that any life-bearing boulder from Earth has ever landed on another planet: the distances to be covered are simply too vast.
But the situation changes radically if, when life-bearing boulders are thrown out of the Earth, they are then eroded into sub-microscopic particles. A particle a few microns in diameter is rapidly thrown out of the solar system by the pressure of sunlight. Fast erosion occurs when a giant comet is thrown into a short-period, Earth-crossing orbit. In that case the mass of the zodiacal cloud is enhanced by two or three powers of ten. A metre-sized boulder, struck at 15 km/sec by the cometary dust particles, may be reduced to micron-sized particles within a decade. Such comets are thrown into suitable orbits every 100,000 years or so and so the process is not uncommon.
The result of this rapid erosion is, not a handful of bodies, but perhaps 1018 particles thrown each year into interstellar space. The Earth becomes surrounded by an expanding biosphere several light years across, containing a vast number of life-bearing dust particles.
The solar system passes through or close to star-forming nebulae occur every 100 million years or so, and these life-bearing particles will be injected directly into protoplanetary systems. In this way, provided that systems with the capacity to support life are generated in the star-formation process, then the necessary codes for replication will already be present. Suppose that we started with a Galaxy in which planets receptive to life are common, but on which life had not taken hold. Then with an initial seeding from the Earth alone, the whole galactic disc would become life-bearing within a few billion years. Life would spread like a chain reaction in a nuclear explosion.
Just as the notion of comets as destroyers of worlds is very old, so also is the notion of them as creators. In 1790, Sir William Herschel was presenting a world view in which comets moved from one star-system to another, replenishing life as they went. There’s nothing new under the sun!
And so it turns out that these rare, large, icy bodies — giant comets — have much more significance for life than we might have expected. One of them may have had a profound effect on the way our ancestors perceived the world around them. They may have been a major factor in the evolution of life on our planet. And they may also have played a vital role in the spread of life throughout our Galaxy.