THE Tunguska blast that flattened a huge swathe of remote Siberian forest one morning in 1908 might have come from within the Earth rather than from outer space.
Although the mysterious explosion didn鈥檛 leave a crater, trees over an area of 2100 square kilometres were burnt and flattened. Most researchers agree that the devastating incident was caused by some kind of celestial body disintegrating a few kilometres above the ground,and say the only serious debate about Tunguska is now whetherit was a fragment of a comet or a stony meteorite.
Yet scientists are still arguing over certain unexplained events and the fact that no samples of the impacting object have ever been found. 鈥淚f a group of experts cannot agree for almost a hundred years, it鈥檚 probably a third option,鈥� says Wolfgang Kundt, an astrophysicist from the University of Bonn in Germany.
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That third option is a high-pressure gas explosion, Kundt told a catastrophe conference in London last weekend. He has calculated that a rapid emission of 10 million tonnes of methane would have caused the same devastating effects as an impact. The gas would have been thrown 200 kilometres upwards, triggering a sideways blast of air that knocked down the trees. As it passed up through the ground the gas would have become charged. Hours later the charged cloud would have sparked lightning, ignitingthe gas in a huge fireball.
This theory helps to explain reports of an unusually bright night sky, which started the night before the Tunguska event and continued for several days afterwards. The methane cloud would have caused ice crystals to form up to 500 kilometres above the ground, Kundt says, high enough to reflect sunlight at night. He also points out that researchers once detected an emission of radon gas from the area that lasted 4 hours, suggesting that there could also be other volcanic gases underground. The Tunguska site lies in the middle of an ancient volcanic eruption zone. But those who favour a cosmic impact shrug this off as a coincidence.
Vladimir Epifanov, a geologist from the Siberian Research Institute of Geology, Geophysics and Minerals, presented a similar theory at a recent conference in Moscow. He adds that the cooling as the escaping gas rapidly lost pressure could have protected the region closest to the explosion from the heat, explaining why some trees near the blast centre were not burnt.
Gas explosions have happened before, though never on such a huge scale. In 1994 in Cando, Spain, a blast formed a shallow crater and toppled 20-metre trees. This was originally attributed to some kind of impact but was later found to be a gas eruption.
Andrei Ol鈥橩hovatov, an independent Russian physicist who is intrigued by the Tunguska event, agrees that the impact theory leaves too many unanswered questions. He points out that witnesses reported strange weather and increased seismic activity in the area for days beforehand. Astronomers counter that an increase in tremors before the event might simply confirm that the planet was passing through a stream of rocky debris, the largest chunk of which fell on Tunguska.
However, there remains one bizarre piece of evidence that no theory can yet explain. Geologists who tried to carbon-date the soil at Tunguska found it so enriched with carbon-14 that it shows up as a future date. The only cause they can suggest for such a signature is a nuclear bomb or engine, but 1908 was far too early for Russia to carry out a nuclear test. 鈥淢aybe it was a spaceship,鈥� jokes Lars Franz茅n, a geologist who has taken samples at the site.
鈥淎ll these theories suffer from a lack of evidence,鈥� admits Benny Peiser from Liverpool John Moores University. But he still thinks a cosmic impact is the most likely explanation. 鈥淲e see impacts in the upper atmosphere every year. We would expect one to hit eventually.鈥�
