Fragmented comet impact

Updating Comets

A fragmented comet impact

Cosmic traffic accidents were the main news story when The Comet is Coming! first appeared in 1980. The collision with a comet or asteroid that wiped out the dinosaurs – then newly indicated – was certainly not the only event of its kind, as my book stressed at the time. Now Prof. Bill Napier at Cardiff University’s Astrobiology Centre suggests that 12,900 years ago the Earth strayed into a trail of fragments from a very large but disintegrating comet. And that encounter, he proposes, caused a drastic change of climate.

Hubble image (2006) of a broken chunk of Comet Schwassmann-Wachmann 3 dividing into dozens of “house-sized” fragments. NASA, ESA, H. Weaver (JHU/APL), M. Mutchler and Z. Levay (STScI)

Comets certainly can break up, and Napier interprets meteor streams of the Taurid Complex as traces of a large comet that entered the inner Solar System system 20,000-30,000 years ago. His idea is that the Earth later ran into a dense trail of its material over the course of an hour, with North America bearing the brunt and experiencing thousands of fireball explosions each comparable with a nuclear bomb.

That would explain a layer of soot found at many sites in the USA, telling of continental-scale wildfires, together with microscopic diamonds of the type produced by meteoritic shocks. And 35 genera of North American mammals were wiped out. All this is persuasive.

Napier also suggests that the impacts provoked a major cooling that lasted 1300 years. The event is well-known to palaeoclimatologists as the Younger Dryas, which takes its name from a flower, Dryas octopetala or mountain avens, which likes cool conditions. The climatic downturn occurred just as the world was emerging from the most recent Ice Age. It was a miniature ice age of its own, with temperatures falling by perhaps 8 deg. C and glaciers re-advancing. Drastic regional changes of climate drove people in the Middle East to invent agriculture, just to survive.

Napier’s multi-impact event coincided with the start of the Younger Dryas. Although you can imagine that impact debris in the stratosphere and smoke from the wildfires would have caused a dimming of the Sun for months or perhaps years, it’s not at all clear why the effects should have persisted for 13 centuries. Napier doesn’t discuss the climatology in his report for the Monthly Notices of the Royal Astronomical Society.

Some climate theorists will offer to explain the Younger Dryas by melt-water from the receding ice sheets stifling the North Atlantic circulation. For me, it’s no coincidence that the Younger Dryas was a time of intense cosmic radiation associated with a long period of weak solar activity. But having faced a lot of flak myself, back in 1980, for “buying” the dinosaur impact story, I’ll not scorn Napier’s latest idea, which is astronomically convincing. His impacts might well have helped to trigger, accelerate and/or intensify the Younger Dryas, even if the depth and persistence of the cooling may need more explaining.

By the way, it’s amusing to note that not until March 2010, 30 years after The Comet is Coming!, did an international panel of 41 experts solemnly pronounce in Science (the Schulte reference below) that a cosmic impact really did do for the dinosaurs. They disposed of the only surviving geophysical hypothesis, namely that massive flood-basalt volcanism in India (the Deccan traps) caused the mass extinction 65.5 million years ago. The main Deccan eruption phase was half a million years too soon. That finding also rules out a proposed compromise between the hypotheses, namely that shock waves from the impact might have triggered the flood-basalt event.

Schulte et al. said without discussion that the impactor was an asteroid. The celebrated astrogeologist Eugene Shoemaker would have begged to differ. He estimated that whatever made the Chicxulub crater in Mexico was 15 kilometres wide and he noted that no asteroids as large as that are known in the Earth’s vicinity. As reported in Magic Universe:

“Not impossible that it was an asteroid,” Shoemaker said. “But if you’re a betting man like I am, you put your chips on a comet. My own hunch is that we will find that the impact of large objects, probably chiefly comets, has profoundly influenced the evolution of life.” That was Shoemaker’s last word on the subject. Soon after recording it for television, in 1997, he lost his life in a car smash in Australia while prospecting other impact craters.

References

W.M. Napier, “Palaeolithic extinctions and the Taurid Complex”, Mon. Not. R. Astron. Soc., in press, released online 1 April 2010.

Peter Schulte, et al., “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary”, Science, Vol. 327, pp. 1214 ff. 2010

Nigel Calder, “Impacts” in Magic Universe, OUP 2003

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