published on 2 July 2008 in space

Meteorite impact

The Tunguska mistery
100 years ago, at 7 o’clock in the morning of the 30th of  June 1908, in a isolated region of Siberia, along the Tunguska river, northwest of the Baikal Lake, one of the most mysterious events of the XXth century happened: a huge explosion was heard in the range of 1000 kilometers, and, because of the dust released in the air, the european nights  were unusually bright  and the sunlight was more feeble.
For decades this event has been incomprehensible to scientists and a source for far fetched presumptions by many people. Only now we are understanding what really happend there one century ago.
Initial studies
Only in 1921, because of communication difficulties and of the tormented historic period in Russia, the geologist Leonid Kulik tried to reach the location and investigate on the event. What he discovered was unsettling: 2,000 Km2 of the forest had been burned and knocked down, but in the center of the distroyed area there were no craters, actually in that very spot the trees were still standing, even if they were burned and stripped of their bark. Interviewing witnesses, Kulik gathered statements which recall the ones on the atomic explosions in Japan 25 years later: the  Russian and Siberian natives that witnessed the explosion spoke about the sky having suddenly caught on fire, about a very strong wind that knocked down trees and sent people flying, about a hot flash that burned their skin and set their clothes on fire, followed by a series of deafening thunder noises very similar to those made by artillery during a battle.
What happened in Tunguska?
Kulik was inclined to believe in the fall of a meteorite, but the absence of a crater made that hypothesis less realistic. In those days and until a few years ago, it was common belief  that meteorites would either fall to earth creating craters and causing destruction according to their size, or  else burn in the atmosphere without  damaging anything. Particularly metallic meteorites, which are very rare, would reach earth, while the rocky ones ( which are 98% of the ones that impact the planet), because they are fragile and made of rock,  have been able to reach earth only in those very rare cases when their diameter reached 50 meters or more. In all other cases they burn when entering the atmosphere.

Alternative hypothesis
This theory on meteorites brought on many alternative suppositions to explain what happened in Tunguska. A far fetched idea that had little to do with the laws of physics, such as the impact of a lump of antimatter, or even the collision with a black hole, the explosion of a natural “hydrogen bomb”, disguised as a comet enriched with deuterium (the H bomb exploding agent) compressed by the heat of friction with the air, the explosion of a natural underground methane deposit and even an accident of an alien space ship, of which however no wreckage had been found.

The latest research
To clear our ideas a bit we had to wait until these last years when, thanks to the progress in the studies on meteorites and to the possibility of making computer simulations, we started to understand that the rigid scheme by which the arrival of these bodies was presumed, was wrong. A few years ago NASA’s Christopher Chyba and Kevin Zahnle did some computer simulations and rebuilt the probable dynamics of what took place 100 years ago. According to them, a 20 – 30 meter wide meteorite made of rock, entered the earth’s atmosphere over Siberia at a speed of 15 km/sec. When it encountered dense air, 8 kilometers above earth, all its kinetic energy transformed into heat, causing a sudden vaporization of the rock and an explosion equivalent to  the one of a 15- megaton atomic bomb, 1000 times more powerful than the atomic bomb of Hiroshima. The heat and dilatation of the air caused by an event of this size, are more than enough to explain the thermal and mechanical destructive effects observed on earth, without being struck by even the tiniest shard of rock.
To confirm the theory that small rock meteorites can behave differently from what was thought in the past and damage the planet, we have the case of the meteorite that fell in Peru in September 2007. Also in this case the meteorite was very small, with a 2 to 5-meter diameter and its composition of  fragile rock which should have caused for it to be shattered in the air. According to Peter Shultz, a meteorite specialist from Brown University “the only possible explanation for such small and friable meteorites to reach the earth’s surface is that the shock wave regathers the rock fragments to the main body of the meteorite, giving it a more aerodynamic shape. With the knowledge that we have gathered from this event and these suppositions, I think we should re-examine once again the origin of many craters and lakes on earth. Even in the Tunguska case there might be a lake to study again.

From the italian expedition…
In the summer 2007 an expedition of geologists from the University of Bologna led by Luca Gasperini, returned stating that they had discovered a crater made by a meteorite fragment responsible for the huge explosion in 1908. It is what we call today Cheko lake which has a 500- meter diameter, 8 kilometers from the center of the explosion. According to Gasperini and his colleagues, there are three things that proved their hypothesis: sediments on the bottom of the lake which are compatible with those that we would have expected to find following the sudden melting of several tens of meters of permafrost 100 years ago; the shape of the lake; the fact that this lake has never been mentioned  in maps or documents dating before 1908. New expeditions in the coming years will try to search for the actual meteorite fragment which may be enbedded in the ground under the lake.

Written by Videoscienza

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