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 user 2007-06-26 at 11:16:00 am Views: 63
  • #18039

    Team makes Tunguska crater claim
    Scientists have identified a possible crater left by the biggest space impact in modern times – the Tunguska event.
    blast levelled more than 2,000 sq km of forest near the Tunguska River
    in Siberia on 30 June 1908.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.

    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
    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.

    impact cratering community does not accept structures as craters unless
    there is evidence of high temperatures and high pressures. Gareth
    Collins, Imperial College London

    “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.”

    Tree observation
    Collins pointed out that the Cheko feature was “anomalously” shallow
    and lacked the round shape of most craters – being more elliptical in
    its form. Elliptical craters only occur if the impactor’s angle of
    entry is less than about 10 degrees.”We know from modelling of the
    Tunguska event that the angle of entry must have been steeper than
    that,” Dr Collins told BBC News.A key feature of other impact craters
    is conspicuously missing from Lake Cheko – a “flap” around the crater
    rim of upside-down material tossed a short distance from the crater by
    the impact.Dr Collins added that if pieces of the space rock had
    survived the airburst, they would have been too small and travelling
    too slowly to have generated a crater the size of Lake Cheko.An impact
    would also have felled trees all around the crater, said the London
    geologist, yet there appeared to be trees older than 100 years still
    standing around Lake Cheko today.Computer models carried out by other
    teams suggest that centimetre-sized fragments of the body could be
    found hundreds of kilometres away from Tunguska.As the impactor plunged
    through the atmosphere, it pushed air out of its way, leaving a
    near-vacuum in its wake.As it broke up, fragments would have expanded
    back up the vacuum and rained out over a much larger area.

    Drill project
    Italian researchers argue that some of the lake’s anomalous features
    could be explained if a space rock was travelling at a low speed and
    had a “soft” impact into the swampy Siberian taiga.The crater could
    have become subsequently enlarged by the expulsion of water and gas
    from the ground.The Bologna team says this could also account for the
    limited damage to the surrounding area and the absence of a rim of
    upturned ejecta.”If formed during the impact, [the rim] would have been
    rapidly obliterated by collapse and gravity-failures during the
    subsequent degassing phase,” the authors write in the journal Terra
    Nova.Lake Cheko does not appear on any maps before 1929, though the
    researchers admit the region was poorly charted before this time.The
    University of Bologna team plans to mount another expedition to the
    Tunguska region in summer 2008.The researchers aim to drill up to 10m
    below the lake bed to the anomaly picked up in the geophysics survey
    and determine whether it really is a piece of extraterrestrial rock.