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PROPHETS of doom have another disaster to tout. If a large comet or
asteroid smashes into Earth, a searing 鈥渦ltraviolet spring鈥� would follow the
devastating 鈥渋mpact winter鈥�.

An asteroid ripping through the atmosphere would ionise nitrogen and oxygen.
The nitric oxide that forms then destroys the stratospheric ozone that shields
us from damaging ultraviolet light. Dust thrown up into the atmosphere would
protect the surface for a while, but once it had settled, survivors would face
elevated ultraviolet levels that could suppress photosynthesis and damage DNA,
says Charles Cockell of the British Antarctic Survey.

Other researchers had suggested that impacts would destroy ozone, but Cockell
and Andrew Blaustein of Oregon State University in Corvallis are the first to
calculate the extent of the damage. They found that a 10-kilometre asteroid
would destroy more than 85 per cent of the world鈥檚 ozone. Dust from the impact
would settle in a year, but the ozone would recover more slowly, exposing the
surface to high ultraviolet levels one to three years after the impact.

Peak ultraviolet levels would more than double, but the increase would be
greatest at shorter wavelengths which have more devastating effects. This would
threaten plants and animals already stressed by the long, dark impact winter.
鈥淒NA damage could be a thousand times higher than we have today,鈥� says
Blaustein.

The Earth probably had a lucky escape when an asteroid hit 65 million years
ago. It struck sulphate-rich anhydrite rock, which covers only 1 per cent of the
Earth鈥檚 surface. That generated ultraviolet-absorbing sulphate aerosols that
lingered in the atmosphere for nearly a decade, allowing amphibians to survive
the event that killed the dinosaurs.