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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 can鈥檛 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 鈥済reatly overstated鈥�.

The Chicxulub impact spread debris across the globe, which settled to form a
layer averaging 3 millimetres thick鈥攖hat鈥檚 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鈥攄roplets that
condensed from vaporised rock. Only the remaining 1 per cent of the debris
consisted of rock pulverised directly into dust.

It鈥檚 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鈥檚 only 100 million
tonnes鈥攁bout 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鈥檛 convinced that the impact produced so little
dust. Jan Smit of the Free University in Amsterdam points out that volcanic dust
isn鈥檛 formed in the same way as impact dust, so the particle sizes wouldn鈥檛
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鈥檛 rest easy just yet. 鈥淥ther things will get
you,鈥� says Brian Toon, an atmospheric scientist from the University of Colorado
in Boulder. He believes the effects of an asteroid impact would be
apocalyptic鈥攆illing the entire sky with fiery meteors as the debris rained
back down onto the atmosphere. 鈥淓verything on the surface is going to catch
fire,鈥� he predicts.

But despite all the debate, much still depends on guesswork. 鈥淲e know so
little about impacts,鈥� says theoretical geophysicist Jay Melosh of the
University of Arizona. 鈥淭he uncertainties are at least a factor of five.鈥�