Video: A visit to the site of the Tunguska explosion
I’M PEERING out the window of a Soviet-era Mi-8 cargo helicopter that’s hovering 50 metres above Lake Cheko, deep in the heart of the Siberian taiga. This is truly the middle of nowhere: from London it has taken me three flights, two days and a pinch of luck to get here. The nearest city, Krasnoyarsk, is some 600 kilometres to the south-west. Pine and larch forests stretch as far as the eye can see in every direction, a view interrupted only by the serpentine wend of the Kimchu river, which runs like an artery through this wilderness.
From the nearest town, Vanavara (population 3000), there are three ways to reach Lake Cheko: a two-day journey up the river, an even longer trek through swamp and forest, or by air. It’s just a short hop in a helicopter, but hiring one costs a princely $4000 an hour, so instead I manage to convince the pilot of a scheduled flight connecting local villages to make a slight detour over the lake. The other passengers – a local family and a geologist prospecting for gas in the area – seem bewildered by my interest in this remote spot.
Unfortunately, it’s too late in the year to land near Cheko – the permafrost has thawed, turning the surrounding area into a swamp. So the pilot dips down and we make a second lap, coming in even lower. It’s a long way to come for a fly-by, but then this is no ordinary lake. If a team of Italian researchers are right, buried in its sediments is an extraterrestrial body which could solve a mystery that has confounded scientists for a century.
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A hundred years ago this week, on 30 June 1908, a giant explosion lit up the dawn sky in the northern hemisphere. Centred near the Tunguska river in western Siberia, the blast knocked down trees over an area of more than 2000 square kilometres (see Map). Eyewitnesses reported intense heat and columns of fire shooting through the sky. Many Evenki, a tent-dwelling nomadic people indigenous to the area, told of animals, their homes and even fellow tribespeople being hurled into the air by a shock wave. An unfortunate few were incinerated. Light from the blast was visible as far west as England and, according to (vol 78, p 228), Londoners could still read their newspapers by the mysterious light in the night sky a day or so later.
Since then, researchers have struggled to explain what caused such a powerful explosion – later calculated to have released 1000 times more energy than the bomb dropped on Hiroshima – let alone the strange lights in the sky or the patterns of destruction on the ground. Suggestions have ranged from an enormous release of gas to the sudden appearance of a miniature black hole to a visit by aliens. The prevailing belief is that the Tunguska event was caused by a bolide, a celestial body such as a comet or asteroid, violently disintegrating high in the atmosphere. Yet there is still no conclusive evidence to support this, or any of the other theories. So this week hundreds of scientists are gathering at a conference in Moscow to take stock and thrash out conflicting ideas. To an outsider, it’s all a bit puzzling: just why, 100 years on, are we still no closer to unravelling this cataclysmic mystery?
It’s certainly not for want of trying. Immediately after the event, seismologists used shock wave data to pinpoint the location of the blast. Yet it wasn’t until 1921 that Leonid Kulik, a Russian mineralogist working in the region, deduced from eyewitness accounts that the blast must have been the result of an iron meteorite smashing into the ground. He returned in 1927 with the first scientific expedition to the area and found thousands of burnt and flattened trees lying in a radial pattern. He used this to infer that the explosion must have taken place at a spot about 65 kilometres north of Vanavara. Since there was no sign of an impact crater, he assumed that it had been swallowed up in the swampy ground. Over the next few decades, Kulik and others returned several times to the area to search for dust and debris from the bolide, but found nothing.
After the second world war, new theories began to emerge. In 1961, Russian geologist Kiril Florenskij suggested that the bolide was more likely a rocky asteroid or a comet made of dust and ice. Such a body would not have survived a journey through the atmosphere. Somewhere, probably between 5 and 10 kilometres above the ground, it would have exploded, creating a huge shock wave that started fires and flattened the trees below. With no solid core to hit the ground, there would have been no impact crater.
To test this theory, researchers began to search for any smaller remnants of extraterrestrial material that might have survived. Every summer, they scoured the swamps for tiny chunks of unusual rock or other signs that might point to a high-altitude blast. Nothing was found.
One of those researchers was Giuseppe Longo of the University of Bologna in Italy. Then, in the late 1990s, Longo stumbled upon a Russian paper published in 1960 speculating that Lake Cheko could be the missing impact crater. Other researchers had dismissed the idea on the grounds that the sediments in the lake were too deep to have been formed in less than 100 years. Nevertheless, the idea caught Longo’s interest, and in 1999 he teamed up with Luca Gasperini and Enrico Bonatti from the Institute for Marine Geology in Bologna to launch an expedition to study the lake for themselves. During a trip to the region in July 1999, the team took sediment samples from the top 2 metres of the lake bed and used sonar to survey it.
What they discovered added to the mystery. Most lakes in the area are flat-bottomed. But the Italians discovered Lake Cheko has a conical shape, which at its centre is 50 metres deep. This is telling, Gasperini says, because besides an impact, “there are not many other ways such a lake could form”. Their sonar survey showed up an even bigger surprise: an intense reflection from an area of sediment 10 metres below the centre of the lake bed (see Diagram). Could this, they wondered, be a remnant of the bolide – or at the very least, an area of sediment compacted by its landing?
The pattern of felled trees observed by Kulik seems to confirm that the air blast occurred about 8 kilometres south-east of Lake Cheko. So Gasperini and his colleagues believe that the lake was created by a fragment of the original bolide that continued on roughly the same north-westerly trajectory after detonation. Travelling at an angle of around 45 degrees, it would have landed in the Kimchu river and formed a crater that soon filled with water. According to their calculations, a rock roughly 10 metres across and moving at a speed of between 1 and 10 kilometres per second would leave a crater about half the size of today’s lake. As it filled with water, the permafrost around the crater would have melted, releasing gases such as methane and enlarging the lake to its current size. In the process this created a sediment layer thicker than would be expected in a lake so young. Gasperini and his colleagues published in the journal Terra Nova late last year (vol 19, p 245). Case closed, you might think.
Far from it. A few months later, a team led by Gareth Collins, a geophysicist at Imperial College London, published a of the Italians’ theory (Terra Nova, vol 20, p 165). “I don’t believe there is any way it can be an impact crater,” he says.
Collins points out that, as an impact crater, Cheko should be surrounded by a raised ring of debris. “At the very least the area should be covered with the earth ejected by the impact,” and there is no evidence of this around Lake Cheko, he says. He also points out that the lake is surrounded by trees that are much older than 100 years old. These should either have been felled by the shock of the impact or buried by the ejected earth. He also reckons that such an event simply could not have happened. Anything as big as 10 metres across would have disintegrated long before it reached the ground, he says.
Anecdotal evidence seems to back Collins’s view. Locals say Lake Cheko existed before 1908, which would rule the theory out point blank. Ludmila Luganova, the director of the national park containing the site, says among the Evenki it is well known that the lake existed prior to 1908. Luganova signs the permits for every visitor to the park from her office in a small wooden building on a back street in Vanavara. She has visited the site several times and met nearly everyone that has been there in the past decade. She’s proud of the park and shows me a collection of fossils she’s found, including a mammoth jawbone and a shell. But she rolls her eyes when I mention the Italians’ theory. “There’s no way it’s an impact crater,” she sighs. And from my vantage point in the helicopter it certainly doesn’t look like a crater. There’s no upturned rim, and it looks much like the hundreds of other oxbow lakes that dot the region. Besides, says Collins, Cheko has an elliptical shape, while most impact craters are circular.
To others, these arguments illustrate a basic problem with the bolide hypothesis. “There is no evidence of any extraterrestrial material,” says Wolfgang Kundt, a physicist at the University of Bonn in Germany. “It can’t be a meteorite impact. That means there’s got to be a terrestrial explanation.”
Kundt believes that the explosion was the result of a massive escape of high-pressure gas from deep within Earth’s crust. His calculations suggest that the sudden release of 10 million tonnes of methane-rich natural gas would create the devastation seen at Tunguska. There is evidence of similar releases of methane on the seabed off Norway, where one crater or “pockmark” on the Blake Ridge covers 700 square kilometres.
“The sudden release of 10 million tonnes of methane-rich gas would create the devastation seen at Tunguska”
It is known that large gas deposits lie beneath the region, hence the interest of my geologist co-passenger. If high-pressure methane escaped at supersonic speed through weak points in the crust, it would have created a shock wave capable of flattening trees across a huge area. This could also explain the strange lights seen across the northern hemisphere for several days after the event: as the gas escaped, it could have become charged. This electric charge would have ignited the methane and it might have burned for days at high altitude, Kundt says.
Jason Phipps Morgan, a geophysicist at Cornell University in Ithaca, New York, has been thinking along similar lines. He became interested in the Tunguska event after reading an abstract of a paper written by Russian researchers that reported the discovery of shocked quartz crystals around the area where the blast happened. Shocked quartz is created when a crystal is struck by a high-pressure shock wave, leaving a telltale deformation in the crystal’s structure. It was thought that only bolide explosions, meteorite impacts and nuclear explosions were capable of producing sufficient pressure. Morgan suggests that a shock wave from a supersonic gas ejection could also do the job.
Morgan describes the process as a “Verneshot”, a volcanic eruption of carbon dioxide, though such an event has never been proved to have happened. As magma rises through Earth’s crust, it cools, releasing carbon dioxide which accumulates in pockets as far down as 80 kilometres below the surface. When the crust can no longer contain the pressure, the gas will burst out, sending rock and gas high into the air. Morgan suggests a Verneshot was responsible for the Permian-Triassic mass extinction about 250 million years ago, which led to the rise of the dinosaurs (New Scientist, 8 May 2004, p 32).
There is some geological evidence to back up the “earth-belch” theory: at the site of the Tunguska blast is an ancient volcano about 10 kilometres across, called the Tunguska Depression. But if Morgan is correct, there should be some evidence where the gas escaped. In fact, the entire region is riddled with vertical structures, analogous to kimberlite pipes, through which volcanic upwellings of magma and gas are vented. Morgan speculates that Lake Cheko may actually be the mouth of one of these structures.
Soon he may have evidence to bolster his theory: Morgan plans to visit the region next week to search for his own samples. “If shocked quartz indeed exists in the Permian sandstone exposed in the Tunguska Depression, then I would be willing to bet a good bottle of wine that the 1908 Tunguska [event] was an earth burp,” he says.
However, Mark Boslough, an authority on asteroid and comet impacts at the Sandia National Laboratories, New Mexico, remains unconvinced. “These are only speculations and statements that are nothing more than opinion,” he says. Boslough has no doubt that an air burst was responsible for Tunguska, and recently published the results of a computer model of the event which, he says, fits with the available evidence and leaves no need for alternative explanations (New Scientist, 8 December 2007, p 10). He calculates that the object responsible could have been just 30 metres across. Gasperini also dismisses Kundt’s explanation. “I feel this theory is totally unsubstantiated,” he says.
Where does all this leave the Italians’ impact hypothesis? In to Collins’s criticisms, Gasperini and his colleagues admit they have no smoking gun, but say many of the criticisms are down to interpretation. Among other things, they argue that the lake crater lacks an obvious rim because the swampy mud is soft, and there is no ejected material because the impact force was absorbed by the yielding ground.
Gasperini also plans to roam the Tunguska site this week to take samples in preparation for a major expedition next year. He wants to drill into the lake bed to sample the mysterious reflector. “If they drill and find extraterrestrial material, then I believe the reflector is real and that it was an impact crater,” says Morgan. “But I’m not convinced there is a reflector there.” Kundt is sceptical. “I don’t think they’ll find anything,” he says.
So the bolide proponents have no evidence of anything extraterrestrial, but the earth-burp camp have no kimberlite pipes. At the moment it’s stalemate, but perhaps the events of the centenary will change that.
Back in Vanavara, Luganova is happy for the mystery to persist. She says the town is preparing for the hordes of Tunguska enthusiasts who will arrive this week to explore the site and no doubt return to town to speculate in the Meteorite bar over a few vodkas. “We don’t have much here; if visitors come it will be good for the local economy,” she tells me. And an unsolved mystery is always good for that.
