Asteroid impact hazards "greatly overstated" Jeff Hecht New Scientist, 2002-01-30
If A collision with an asteroid is going to finish us off, it will have to be a lot larger than anyone thought, according to a controversial new study of the impact that wiped out the dinosaurs. Virtually everyone agrees that the asteroid that hit Chicxulub in Mexico 65 million years ago killed the dinosaurs, but how it did so is unclear. A long-standing theory is that clouds of dust hung in the upper atmosphere for months, blocking sunlight and stopping plants growing. But no one is sure that this is really the reason, and finding out is critical for assessing the risk asteroids pose to humanity. Now geologist Kevin Pope of Geo Eco Arc Research in Aquasco, Maryland, is claiming that dust cannot have been to blame. Only dust grains smaller than a micrometre across stay suspended in the atmosphere, and Pope says that the 10-kilometre asteroid would not have created enough fine dust to have a global effect. Instead he thinks sulphur from the rocks vaporised by the impact may have formed sulphate aerosols that blocked out the light. He says earlier overestimates of dust levels mean that the hazards from an asteroid impact today have been "greatly overstated". Particle uncertainty The Chicxulub impact spread debris across the globe, which settled to form a layer averaging 3 millimetres thick--that's a few trillion tonnes of material. But having reviewed previous work on the subject, Pope says that more than 99 per cent of the layer is made up of spherules--droplets that condensed from vaporised rock. Only the remaining 1 per cent of the debris consisted of rock pulverised directly into dust. It's still uncertain what the size distribution of that dust would have been, but from studies of volcanic dust, Pope deduces that less that 1 per cent of it consisted of particles smaller than 1 micrometre. That's only 100 million tonnes--about 10 times as much dust as was released by the 1991 eruption of Mount Pinatubo, which had a barely measurable effect on global climate. But other researchers aren't convinced that the impact produced so little dust. Jan Smit of the Free University in Amsterdam points out that volcanic dust isn't formed in the same way as impact dust, so the particle sizes wouldn't necessarily be the same. He says his studies of iridium in the impact layer suggest that at least half of it is in particles smaller than 0.1 micrometres. Even if Pope is right, we can't rest easy just yet. "Other things will get you," says Brian Toon, an atmospheric scientist from the University of Colorado in Boulder. He believes the effects of an asteroid impact would be apocalyptic - filling the entire sky with fiery meteors as the debris rained back down onto the atmosphere. "Everything on the surface is going to catch fire," he predicts. But despite all the debate, much still depends on guesswork. "We know so little about impacts," says theoretical geophysicist Jay Melosh of the University of Arizona. "The uncertainties are at least a factor of five." Journal reference: Geology (vol 30, p 99) =================== --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] Visit IAGI Website: http://iagi.or.id IAGI-net Archive 1: http://www.mail-archive.com/iagi-net%40iagi.or.id/ IAGI-net Archive 2: http://groups.yahoo.com/group/iagi ===================================================================== Indonesian Association of Geologists [IAGI] - 31st Annual Convention September 30 - October2, 2002 - Shangri La Hotel, SURABAYA
