TEAM MAKES TUNGUSKA CRATER CLAIM

[Anonymous]

Team makes Tunguska crater claim
Scientists have identified a possible crater left by the biggest space impact in modern times – the Tunguska event.
The
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
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. 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
Dr
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
The
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.

[Author]

June 26, 2007 at 11:16 AM