3-D reconstruction (viewpoint from S) of the morphology of the Lake Cheko based on real topographic/bathymetric data. The water level has been placed 40 m below the actual level to enhance underwater morphological features.
A comet or asteroid is thought to have exploded in the Earth's atmosphere with a force equal to 1,000 Hiroshima bombs.
Now, a University of Bologna team says a lake near the epicentre of the blast may be occupying a crater hollowed out by a chunk of rock that hit the ground.
Lake Cheko - though shallow - fits the proportions of a small, bowl-shaped impact crater, say the Italy-based scientists.
Their investigation of the lake bottom's geology reveals a funnel-like shape not seen in neighbouring lakes.
In addition, a geophysics survey of the lake bed has turned up an unusual feature about 10m down which could either be compacted lake sediments or a buried fragment of space rock.
Shocking rocks
Luca Gasparini, Giuseppe Longo and colleagues from Bologna argue that the lake feature, about 8km north-north-west of the airblast epicentre, may have been gouged out by remnant material that made it to the ground.
"We have no positive proof this is an impact crater, but we were able to exclude some other hypotheses, and this led us to our conclusion," Professor Longo, the research team leader, told BBC News.
The object that hurtled through the atmosphere on the morning of 30 June, 1908, is thought to have detonated some 5-10km above the ground with an energy equivalent to about 20 million tonnes of TNT. The explosion was so bright it even lit up the sky in London, UK.
Small fragments of the body should have survived the airburst and made it Earth. But, mysteriously, no crater - or even the slightest trace of the impactor - has ever been positively identified.
The impact cratering community does not accept structures as craters unless there is evidence of high temperatures and high pressures.
"In my opinion, they certainly haven't provided any conclusive evidence it's an impact structure," commented Dr Gareth Collins, a research associate at Imperial College London, UK.
He added: "The impact cratering community does not accept structures as craters unless there is evidence of high temperatures and high pressures. That requires evidence of rocks that have been melted or rocks that have been ground up by the impact."
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The BBC
Comparison between the morphology of (a) Lake Cheko and Lake Nikolaji (a typical thermokarst lake; Schwamborn et al., 1998), and (b) the Odessa Impact Crater in Texas (Maysell, 2004).
L. Gasperini, F. Alvisi, G. Biasini, E. Bonatti, G. Longo, M. Pipan, M. Ravaioli, R. Serra
A possible impact crater for the 1908 Tunguska Event
3-D image of Lake Cheko (viewpoint from SW) obtained using aerial photographs collected during our Tunguska99 expedition. The catamaran used for the geophysical survey and core collection is also visible. (a) Close-up view of the lake near the W shore, where a circular feature marking gas-escape from the lake bottom is visible.
Abstract
The so-called �Tunguska Event� refers to a major explosion that occurred on 30 June 1908 in the Tunguska region of Siberia, causing the destruction of over 2000 km2 of taiga, globally detected pressure and seismic waves, and bright luminescence in the night skies of Europe and Central Asia, combined with other unusual phenomena. The �Tunguska Event� may be related to the impact with the Earth of a cosmic body that exploded about 5�10 km above ground, releasing in the atmosphere 10�15 Mton of energy. Fragments of the impacting body have never been found, and its nature (comet or asteroid) is still a matter of debate. We report results from the investigation of Lake Cheko, located ~8 km NNW of the inferred explosion epicenter. Its funnel-like bottom morphology and the structure of its sedimentary deposits, revealed by acoustic imagery and direct sampling, all suggest that the lake fills an impact crater. Lake Cheko may have formed due to a secondary impact onto alluvial swampy ground; the size and shape of the crater may have been affected by the nature of the ground and by impact-related melting and degassing of a permafrost layer.
