Science: Fireball over Siberia: 1908

Have the Soviets demystified the big blast that leveled a forest?

An alien spacecraft that exploded while trying to land on earth. A hit by a stray bit of antimatter, or a speeding mini-black hole, or the head of a comet. These are some of the fanciful theories offered over the years to explain the fireball in the sky and the giant explosion that devastated a remote region of Siberia on June 30, 1908, leveling trees for miles around, knocking over huts, stampeding reindeer and creating an enormous shock wave detected around the world.

Now Soviet scientists are reviving a more down-to-earth explanation. After six years of investigation, researchers at the Institute of Geochemistry and Mineral Physics in Kiev have concluded that the blast was caused by something less exotic: a meteorite. Many scientists in the past refused to accept the meteorite theory because there is no trace of any impact crater at the center of the destruction, in the isolated Tunguska area, about 950 km (590 miles) north of Irkutsk.

The Soviets have some convincing evidence for their claim. Using special high-temperature ovens, they incinerated peat collected from the region. In the ashes they found many strange, irregularly shaped and extremely hard black grains, which laboratory examination revealed to be tiny diamonds. Institute Geochemist Emil Sobotovich explained that the little diamonds could only have been created under extraordinarily high pressures. Such conditions deep within the earth produce diamonds, which are brought to the surface in eruptions of molten magma through kimberlite, or volcanic, pipes. But extreme pressures also occur during high-velocity collisions between celestial objects; uralites, a class of meteorites that presumably have been involved in such deep-space impacts, contain such tiny diamonds. Since no volcanic pipes have been identified in the Tunguska area, Sobotovich concluded that the Siberian diamonds were formed far from the earth.

As added evidence that the diamonds arrived in a meteorite, Sobotovich cited their level of carbon 14, a radioactive isotope of carbon found in meteorites that have been subjected to prolonged bombardment by cosmic rays in space. University of Chicago Geochemist Edward Anders cautions that even a trace of unburnt peat could produce a high reading of carbon 14. But, based on these levels, the Soviets calculate that the meteorite must have weighed at least 4,000 tons.

Then why did it not leave a crater? Anders explains that if the meteorite was of the stony type, rather than one made up mostly of iron, it could not have withstood the enormous forces created by its fiery plunge through the atmosphere. It would have exploded before impact, scattering particles over a wide region. The explosion would have produced a fireball and intense shock waves that battered everything on the ground below. Where did the meteorite originate? Slovak Astronomer Lubar Krésak has suggested that it was a chunk of Comet Encke, a periodic visitor to the earth's vicinity that is also the probable source of an annual meteor shower in late June—the time of the Tunguska event.