WHAT do the space shuttle and a giant explosion in 1908 have in common? Both have been followed by strange, glowing clouds, and these may point to what caused the Tunguska blast a century ago.
The blast destroyed a swathe of Siberian forest near the Tunguska river, and a day later people across Europe saw strange clouds that lit up the night sky. These were probably : rare clouds of ice crystals high enough to reflect sunlight long after sunset (see image).
The fact that the clouds were seen just a day later suggests the explosion somehow injected water vapour into the normally dry upper atmosphere. But no one could explain how vapour travelled to the western edge of Europe in less than 24 hours.
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Michael Kelley of Cornell University in Ithaca, New York, now believes the water was carried by high-speed winds in the , a region between 90 and 500 kilometres up, about which little is known. Although such eddies have never been measured, Kelley believes these winds would explain why observers near the poles have seen noctilucent clouds a few days after shuttles have launched from distant locations (Geophysical Research Letters, ).
Kelley thinks the clouds mean that the explosion was caused by a wet, icy comet, which shed some of its moisture up in the thermosphere, rather than by a dry, stony asteroid.