Wolfgang Kundt, a professor of astrophysics at the University of Bonn, dismisses comet and asteroid theories as pseudo-science and suggests an alternative 'volcanic blow-out' scenario for the fireball. As natural gas escaped from narrow underground volcanic vents, it became charged. The escaping gas, which contained mostly methane, raced upwards at high speed and started mixing with air. After a few hours, this charged, volatile mixture sparked lightning that ignited it into a fireball. This massive fireball contained as much as 10 million tonnes of natural gas, and caused the devastation we all know about.
The Tunguska site, according to Kundt, lies in the middle of the 250-million-year-old Kulikovskii volcanic crater, which has several faults or fractures running through layers of rocks. In the Tunguska blast, the gas escaped through a kimberlite, a carrot-shaped volcanic pipe in the rock, which had been formed when hot magma from the Earth's molten mantle pushed upwards under high pressure. Diamonds are formed in kimber-lites more than 150 kilometres beneath the surface, and brought up in volcanic vents or pipes called diatremes. Kimberlites are named after Kimberley in South Africa, where legendary diamond reserves were found in the 1870s. Once thought to be common only in South Africa, kimberlites have now been found in other parts of the world.
The first expedition to the Tunguska site was carried out in 1910 by a wealthy Russian merchant and goldsmith named Suzdalev who, on return, urged the locals to keep silent about his expedition. Apparently, they obeyed. We do not know whether Suzdalev left Tunguska with his sledge filled with sacks of diamonds, but we do know from Kundt that the ejection of gas caused the formation of diamond-rich kimberlite diatreme pipes.
Kundt claims in the journal Current Science that dozens of funnel-shaped holes, including the famous Suslov crater discovered by Kulik, had been 'blown from below' during the morning of the explosion. He cites Evenki eyewitness accounts in support of his claim. He explains the presence of ice crystals in permanently frozen mud in the Suslov crater by saying that 'during its formation water should have intruded into its cavities'.
Russian scientists have explained this phenomenon as the result of permafrost, and have pointed out that similar holes are often found in other parts of Siberia.
Kulik had also found a decayed tree stump at the bottom of the Suslov crater. How could a tree stump make its way to the bottom of a crater blown from below? Kundt explains: 'There have been dozens of trees, standing on top of what is now the Suslov hole. Most of them were hurled to large distances, but one of them may have managed to fall back in, more or less upright. Alternatively, this stump could have slid back from the crater's rim along with transient mud flow.' Kundt believes that his theory has also answered the questions: 'Why did several expeditions find large numbers of detached tree stumps laying around in the Great Cauldron and its vicinity?'; and 'How did they get there?'. He says: 'To me, they are a clear indication of ejections, from the holes at whose surfaces they had grown.'
The pattern of treefall in the blast area, which has been studied by many researchers, has many unique features. The almost radial pattern - 'wiggly rather than straight-line radial', according to Kundt - has five centres and follows the valleys and hills. It shows islands of survived trees in the valleys. It also has 'telegraph poles' near the epicentre, which are reminiscent of the Hiroshima blast waves. Kundt claims that such fine structures of blasting and felling could not be explained by one big explosion on the ground. He considers the impact models inconsistent, as 'all of [them] produce parallel treefall patterns, if properly evaluated'. The actual pattern 'requires several successive localized explosions near the ground', he says.
A detailed study of the treefall pattern made by the 1991 Italian expedition has also suggested more than one centre of the explosion.
Kundt explains 'bright nights' observed in parts of Europe and Asia after the Tunguska explosion by saying that 'such nights are unique in the recorded history except for the 1883 Krakatoa volcanic eruption'. He says that the bright nights of Krakatoa and Tunguska were due to natural gas - mainly methane - which could rise to an altitude of 200 kilometres, where it was reheated by solar radiation and by slow burning with the surrounding atomic oxygen and rose again to an altitude of about 600 kilometres. Water vapour formed during slow burning froze out as snowflakes, which scattered the sunlight and gave rise to bright nights.
Kundt's other reasons against the extra-terrestrial origin of the Tunguska blast include:
■ An asteroid would have left a trace, whereas a comet would have exploded too high, and also would have been discovered before the impact.
■ The absence of any remnants of the interstellar body. An iron comet would have left a large, lasting crater. A stony asteroid would have left either big fragments or at least 4-millimetre-thick dust.
■ Several witnesses reported the sound of gunfire before they saw a 'pillar of fire' in the sky. This order of events is expected from a volcanic blow-out, not from an extra-terrestrial impact.
■ The heat felt by many witnesses at Vanavara, about 70 kilometres from the explosion site, cannot be explained by a meteorite trail. A meteorite trail cannot produce such intense heat because it is narrow; it would have to pass quite near to Vanavara with a rather large speed. 'What counts is the spherical angle of the hot source, seen by your face: you feel the heat of a near bonfire - covering a large spherical angle - but you cannot possibly feel the heat of a (short and narrow!) meteoritic trail', he says.
For his last reason - number 19 on his list - Kundt relies on statistical odds. It is well known to geologists that only a small number of terrestrial craters were gouged in by rocks from space; most of them were formed by volcanoes. In all the faces of volcanism - ranging from hardly noticeable outgassing, through lava flows, to mud volcanoes, real volcanoes and explosive, supersonic ejections - rising natural gas is the primary piston.
There are other reported cases of natural gas explosions, but none as dramatic as the Tunguska blast. In his Current Science paper, Kundt quotes a 1988 incident reported to him by the American geologist Thomas Gold: 'A United Airlines plane on the way from Tokyo to Honolulu in calm air experienced a sharp upward bump followed, in a fraction of a second, by a mightier downward movement with a recorded speed implying a downward acceleration of 4 g.' Gold explained the upward bump as the crossing of a methane cloud rising at high speed. The plane's engines then ignited the methane-air mixture above the plane. This explosion forced the plane downwards and injured many people seriously. The plane had to return to Tokyo to attend to the wounded.
Kundt considers Tunguska 'a lovely detective story which requires a broadly educated physicist's mind for its resolution'. He says that he owes a lot of thanks to Andrei Ol'khovatov (see 'Ghostly geometeors' below), whose sober analysis converted him from the mainstream opinion to a physically consistent one.
After Kundt's presentation at a conference on environmental catastrophes in London in 2002, Jesus Martinez-Frias of the National Aerospace Institute in Madrid (obviously with a 'broadly educated physicist's mind') said that Kundt's geophysical hypothesis was 'a fresh idea'. 'It could be the answer', he suggested.
Like Kundt, Vladimir Epifanov, a geologist from the Siberian Research Institute of Geology, Geophysics and Minerals, also believes that the epicentre of the Tunguska explosion is indeed located just above a major oil and gas field. But he suggests a different mechanism for the explosion: a powerful fluid jet that had suddenly shot up under high pressure from the depths of the Earth.
According to Epifanov, gases from the oil deposits and methane from the coal beds, which had accumulated under a thick cover of basalt, suddenly broke through one day. A moderate earthquake could have initiated the process. The fluid jet was accompanied by dust that created a layer of aerosols in the upper atmosphere. If this layer became charged with electricity, it could have produced the spark that set off the explosion. The fireball then rushed towards the ground, flattening trees in a circular pattern for many kilometres. However, the cooling as the escaping gas rapidly lost pressure could have formed an ice dome around the place where the gas discharged, protecting the trees in the centre of the blast. This would not be the case with an impact from space.
Critics of the 'blast from below' scenario ask: what about the fireball that was seen by many witnesses racing across the Siberian sky from the south-southeast to the north-northwest just before the explosion? A volcanic gas blast fails to explain a racing fireball in the sky. Have these eyewitness accounts flown out of the window - just like a ball of lightning?
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