By Bill Napier, AOP Visiting Astronomer
A comet typically comprises about 50% fine dust embedded in a frozen matrix of water, methane, carbon dioxide and other organic compounds. Approaching the inner planetary system it begins to disintegrate, with fragments splitting off along with dust and gas. The fragments spread around the orbit, forming a meteor stream. If the Earth’s orbit intersects that of the comet, we see an annual meteor shower. There are about ten annual showers in which a dozen or more shooting stars can be seen every hour, and many more weaker meteor showers, merging into the general sporadic background of meteors which cannot be associated with any particular comet.
On occasion, a meteor shower may be so intense that it is best described as a meteor storm. One such event took place on 13 November 1833. The Leonid meteor stream was seen throughout the United States of America with something like 1000 meteors being counted every minute, arriving ‘thick as snow coming down in a snowstorm.’ The source of the Leonid meteors was discovered in 1866: it is a small comet, 3.6 km in diameter, with a 33 year orbital period. The orbit of the meteor stream passes within about a million km of the Earth’s orbit, about 1% of the Earth-sun distance.
The fact that a small comet can produce such a spectacular celestial display raises some pertinent questions: how big can comets be? How often do they come our way? Might there be terrestrial consequences of a large comet disintegrating in our neighbourhood? And do we have evidence for any such events in our past?
Evidence that an exceptionally large comet dominated our interplanetary environment in the relatively recent past is provided by the zodiacal cloud, a disc of dust particles orbiting the sun out to beyond the orbit of the Earth. The dust particles slowly spiral in to the sun, and will typically disappear within 10,000 years. The great bulk of this material comes from the disintegration of comets, and it has long been known that the present-day input of comet dust to the cloud is orders of magnitude too small to be consistent with a steady-state balance. As the comet astronomer David Hughes has said, “… at some time in the last 1,000 to 100,000 years, the cloud has benefited from a large and unusual mass enhancement.”
The zodiacal cloud is not symmetric about the Sun; rather, two thirds of it is in the form of a broad tube. Imbedded in this tube is an ancient, widely dispersed meteoroid stream — the Taurids — which the Earth takes several weeks to pass through each year, as against only a few days for most meteor showers. It contains over a dozen substreams, and is associated with about two dozen known ‘asteroids’, which are most likely to be dormant comets. Comet Encke, with an orbital period of 3.3 years, lies near the centre of this stream and is in a low-inclination orbit. The whole system is known as the Taurid Complex, and is mostly easily understood as the disintegration product of an erstwhile large comet.
Reassembling this material leads to an estimate that the progenitor comet may have been about 100 km across, although with a large uncertainty. Such a body has 50 times the mass of the near-Earth asteroid system, and comets of this size are expected to leak into the inner planetary system every 50,000 years or so. From the spread of the meteoroid material it looks as if the giant progenitor began to disintegrate at least 20,000 years in the past. It should be noted that new constraints from colour measurements and the continuing flow of new asteroid discoveries from telescope surveys point to an alternative picture of a much less massive Taurid complex of asteroids, though still equivalent to an atypically large nucleus for the progenitor comet. However the evidence of a recent large mass enhancement of the zodiacal cloud, and the concentration of material in the form of a massive tube enveloping the Taurids, do seem to indicate a truly exceptional comet.
Although the tail of a comet is its most spectacular feature, the disintegration of most comets proceeds through a series of splittings, with typically hundreds of fragment clusters, comprising short-lived comets and dust, created over its lifetime. The orbits of short-period comets wobble slowly in space, due to the perturbing influence of the giant planets, and the orbits of the giant progenitor and the Earth have intersected several times during the comet’s lifetime. When this happened, there is an enhanced risk of running through fresh clusters of orbital debris, yielding a bombardment of intensity far beyond anything experienced in modern times, thousands of times greater than that of the 1833 meteor storm.
One such encounter may have taken place 12,800 years ago. This was at the onset of the Younger Dryas cooling, a sudden plunge in global temperature which returned the Earth to something like glacial conditions for 1300 years. It has been proposed that this climate change was involved in the extinctions of 35 species of large mammals, including mammoths, and a bottleneck in the human population of that time.
In 2007 a team of Earth scientists identified a carbon-rich black layer, 12800 years old, at many sites across North America. At the base of these layers, they reported charcoal peaks and held that these were evidence for extensive wildfires. They also found sharp concentrations of magnetic grains, nanodiamonds and other indicators which they took as ‘impact markers’, and presented them as evidence for the 10 million megaton impact of a 4 km comet north of the Canadian great Lakes, the heat from the fireball setting North America ablaze. This bold claim unleashed a firestorm of controversy, which has yet to subside.
Since then, other groups have reported the presence of what they hold to be cosmic markers at several sites in North and South America, in Europe and the near East. The Younger Dryas boundary was found to contain peak abundances of magnetic spherules, glassy microscopic spheres, platinum, iridium, nickel, cobalt, microscopic diamonds and shocked quartz, usually synchronous with dramatic changes in vegetation and a sudden disappearance of large animal remains. These markers have now been found at about 40 sites across North America and Europe, and in South America, South Africa and western Asia. Collectively they have no obvious terrestrial explanation, and seem to be indicative of some high-temperature global event. In addition charcoal peaks have been found at the boundary worldwide, indicating a major biomass burning event: a world ablaze.
One objection to the comet impact idea was that such an event, occurring so recently, was highly improbable, something expected perhaps a once in a few hundred million years. However the evidence for a large, disintegrating comet in our neighbourhood changes the picture: the probability of running through a fragment trail is many thousands of times greater than that of being struck by the comet itself. Rather than some highly implausible event, we are dealing with something expected over the timescale involved. The Earth has probably run through a fragment swarm at least once, and possibly a few times, over the active lifetime of the Taurid progenitor. An encounter could easily result in bombardments by fragments with total impact energy of order 10,000 megatons, comparable to that expected from a full-scale nuclear war, and with sufficient smoke generated by incoming meteors to darken the sky, triggering a severe cosmic winter.
Supporting evidence that such an event took place 12,800 years ago has now been forthcoming from an investigation of the prehistoric village Abu Hureyra in northern Syria, a 28-acre site which had been occupied continuously for 6000 years. In the 1970s, the archaeologist Andrew Moore and his colleagues excavated this village shortly before the sluice gates of a new dam were closed and the village was submerged under the waters of Lake Assad. The team rescued two tons of soil, mud bricks, carbonised seeds, plant, animal and human remains, and 30 years later, Moore and his colleagues reinvestigated this material and published their results in 2020. They found a wide range of evidence for a high-temperature event at the onset of the Younger Dryas, along with high concentrations of iridium, nickel and platinum suggestive of cosmic material, mixed with grains of meltglass that could only have been created at temperatures of over 2000 degrees Celsius, about a thousand degrees hotter than could be reached in thatched hut fires or in any process available before the 20th century. The village had in effect been blowtorched from the sky.
Until that time, the people of the near East were hunter/gatherers, but the sudden climate change forced a transition to the cultivation of crops, and hence agriculture, a pre-requisite for the development of civilisation. All of which leads to an interesting speculation: without the giant comet, would civilisation have developed anyway, or would we still be hunting gazelle?
The new research is described in a paper published in the journal Nature.