Page last updated at 15:00 GMT, Friday, 26 June 2009 16:00 UK

Cloud clue in space blast mystery

Noctilucent clouds
Bright skies at night were noted in Britain for days after the event in 1908

There is new evidence in the debate regarding the 1908 Tunguska event that destroyed 80 million trees in Siberia.

Researchers say that clouds that form at the poles after shuttle launches are due to the turbulent transport of water from shuttle exhaust.

Similar clouds were visible at night long after the Tunguska event.

The Geophysical Research Letters study suggests that an icy comet, rather than a meteor, must have been responsible for the event.

The Tunguska event has generated much debate, but 100 years on it remains unclear if it was caused by a comet, an asteroid, or a meteoroid. What is known is that after the event, bright skies at night were noted in a number of places, particularly Britain.

'Unexpected physics'

Night-time or "noctilucent" clouds are the highest in the Earth's atmosphere, forming at an altitude up to 85km. They lead to bright night skies when they are illuminated by sunlight from beyond the horizon.

Such noctilucent clouds were noted in the polar regions by researchers after launches of space shuttles Discovery in 1997 and Endeavour in 2003.

Because the shuttle's main engine combines liquid oxygen with hydrogen, launches produce more than 300 tonnes of water that is deposited in the upper atmosphere.

Tunguska picture (AP)
The event flattened over 2,000 square kilometres of forest

However, it was unclear how a water vapour trail could spread to a 1000km size and travel more than 8000km to the poles.

Now, Michael Kelley of Cornell University and his colleagues propose that so-called "two-dimensional turbulence" is to blame.

The phenomenon arises when, instead of being able to move freely in three dimensions, fluids are constrained by, for example, a magnetic field.

As a result, they can move much more quickly in two dimensions in which they are still free to move.

The team says that the water vapour could get trapped in such a two-dimensional layer, being funnelled to the poles quickly while being spread out over vast distances.

"There is a mean transport of this material for tens of thousands of kilometres in a very short time, and there is no model that predicts that," Professor Kelley said.

"It's totally new and unexpected physics."

Having found a mechanism for the transport of water over vast distances, leading to the noctilucent clouds, the team now suggest that bright night skies after the Tunguska event could be explained by a great deal of water being released into the upper atmosphere.

That suggests the cause was a comet that shed its icy outer coating before plunging to Earth, rather than an asteroid or meteoroid.

"It's almost like putting together a 100-year-old murder mystery," Professor Kelley said.

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