Friday, June 30, 2017

The Tunguska Event—Flashback to Extinction or Omen?

An artist rendition of what Siberian natives saw in the sky before the explosion that has come to be known as the Tunguska Event.

It was the largest impact event on or near Earth in recorded history.  An explosion three to six miles above the surface of the Earth is estimated to have been in the range of 10–15 megatons of TNT—about 1,000 times greater than that of the atomic bomb dropped on Hiroshima.  It knocked down virtually everything standing in 830 square miles, shook the Earth the equivalent of a 5.0 earthquake on the Richter scale, caused fluctuation in atmospheric pressure measured in London, and caused a stratospheric cloud of ice crystals to orbit the Earth for months affecting the climate of the Northern Hemisphere.
But because the explosion on June 30, 1908 occurred over one of the most remote and under populated regions of the earth—an almost unexplored (by Europeans) forest near the Podkamennaya Tunguska River deep in south-central Siberia it was barely noted at the time.  Subsequent events—World War I, the Russian Revolution, and the Civil War that followed meant that the site of the Tunguska Event was not investigated until an expedition was finally mounted in 1927, 19 years after the explosion.
On July 2, 1908 first sketchy reports were printed in a Siberian newspaper:
On the 17th of June [Julian calendar still in use in Russia], around 9 a.m. in the morning, we observed an unusual natural occurrence. In the north Karelinski village [200 verst north of Kirensk] the peasants saw to the north west, rather high above the horizon, some strangely bright (impossible to look at) bluish-white heavenly body, which for 10 minutes moved downwards. The body appeared as a “pipe”, i.e., a cylinder. The sky was cloudless, only a small dark cloud was observed in the general direction of the bright body. It was hot and dry. As the body neared the ground (forest), the bright body seemed to smudge, and then turned into a giant billow of black smoke, and a loud knocking (not thunder) was heard, as if large stones were falling, or artillery was fired. All buildings shook. At the same time the cloud began emitting flames of uncertain shapes. All villagers were stricken with panic and took to the streets, women cried, thinking it was the end of the world.
The author of these lines was meantime in the forest about 6 verst [6.4 km] north of Kirensk, and heard to the north east some kind of artillery barrage, that repeated in intervals of 15 minutes at least 10 times. In Kirensk in a few buildings in the walls facing north east window glass shook.
Scattered reports reached Moscow within days, but received surprisingly scant interest.  No official or scientific investigations were undertaken to find out what the hell happened out there in the boonies.
Soviet Mineralogist Leonid Kulik led expeditions to investigate what had happened twenty years after the fact.
Finally in 1921, as the Civil War was winding down, mineralogist Leonid Kulik was dispatched by the Soviet Academy of Sciences to the Podkamennaya Tunguska River basin not to investigate the incident 13 years early but as part of a survey to discover possible natural resources in the remote area.  He began hearing stories of the mysterious event from locals, and started informally collecting evidence as well as he could without actually traveling to the area of the impact.  He concluded a large meteor or small asteroid had either struck the Earth or exploded on entry relatively close to the surface.
Kulik spent years trying to convince Soviet science authorities to fund a full scale expedition to the site.  But the practical commissars keeping a close eye on those foolish scientists had little interest in abstract science or basic research.  They insisted that some tangible and practical economic benefit must be the result of any investigation.  Finally he convinced them that he might be able to locate a very large nickel/cadmium/iron meteorite, the kind representing some of the densest and hardest stone found on Earth, which could be useful in the Soviet steel industry.  He had to do this with both fingers crossed behind his back knowing that even if such a meteorite had struck the earth, it would have shattered into small fragments spread over a wide area and unlikely to be recoverable in any economically viable amounts.
None-the-less Kulik arrived in the area in 1927 at the head of a well-supplied expedition.  He contacted with local Evenks, indigenous semi-nomadic reindeer herders and hunters, to guide him from remote Russian trading outposts to the site.  Travel through dense forests cut by numerous rivers and streams was extremely difficult.  But after weeks of travel the group neared the target area.  Then, just south of the site, the Kulik guides flatly refused to take him further, fearing possible supernatural beings called valleymen associated with the site.  Kulik had to turn back to a village and arrange for new guides.
Finally the expedition reached a ridge overlooking the impact area.  To Kulik’s surprise, he could detect no discernible impact crater.  Instead around ground zero he found a vast zone more than 5 miles across of trees scorched and devoid of branches, but standing upright.  Trees further from the center were more lightly singed.  Closer in but all knocked over in a direction away from the center was a giant ring of flattened trees radiating from an invisible center.  
In 1927 Kulik found a ring of trees knocked down outward from a suspected epicenter.
Kulik led three more expeditions to the area looking for evidence of an impact.  His best hope seemed to be numerous small pothole bogs which he thought might have been created by meteorite fragments.  But this turned out to be a blind alley.  Draining one turned up an old tree stump at its bottom, not extraterrestrial stone.
The Soviets continued to send teams of investigators to the region for decades but the mystery of just what had happened only deepened.  Eventually microscopic beads of silicate and magnetite were found in the soil, and still later similar beads were found in the resin of some trees.  The beads or spheres also contained significant traces of nickel iron similar in composition to that found in meteorites.  All of this bolstered the opinion that an object from space was involved, but that it had likely been virtually destroyed by an explosion in the atmosphere.  That became the most widely accepted theoretic explanation of the event.
Atmospheric nuclear tests it the ‘50’s and ‘60’s seemed to confirm the hypothesis that an asteroid exploded.  Air bursts over forests showed that trees directly under the blast were stripped as the blast wave moved vertically downward, while trees farther away were knocked over because the blast wave was traveling closer to horizontal when it reaches them.  
Map based on Kulik's observations showing the suspected epicenter, trajectory of the meteor or astroid fragment, and the rings of devastation.  The inset shows the location in the Soviet Union with K representing the impact zone.
By then aerial surveys showed the blast area was actually in the “shape of a butterfly with wings outstretched” occupying an area of 830 square miles, with a wingspan of 43 miles and a body length of 34 miles.  Soviet experiments performed in the mid-‘60s, with model forests and small explosive charges slid downward on wires, produced strikingly similar butterfly-shaped blast patterns  suggested that an extraterrestrial  object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north and had exploded in mid-air. 
Making up for the lack of interest in the first few years, the Tunguska Event continues to arouse and challenge science to come up with new techniques and technologies almost yearly to discovering what happened.  The exploding asteroid theory remains the top contender, but the continuing absence of any fragment of the object has opened the door to other conjectures. 
Among the several theories advanced, the one which picked up the most steam was that instead of an asteroid, the object was a small comet or a fragment of a larger comet that had disintegrated in orbit earlier.   Advanced by some Soviet scientists in the 1930’s, the fact that the head of a  comet—made up of ice particles and space dust exploding in the atmosphere would explain why no physical debris has been found on earth.  Dissipation of the ice crystals into the upper atmosphere could also explain the “glow” that was reported for some days after the event and the orbiting particles that reduced sunlight hitting the Earth over the next year.
In the ‘70’s there Soviets even advanced a candidate, fragment of the short-period Comet Encke, which is responsible for the Beta Taurid meteor shower which coincided with the event.  Later Western research has cast doubt on the comet theory pointing out that a comet reaching the atmosphere at the low angle expected would have exploded or vaporized far earlier and not nearly reached the surface, if a handful of miles can be said to be near the surface.  Other research showed that the object came in a direction from the asteroid belt.
Lake Cheko thought by some to have been formed from the impact crater.
If the comet idea was doubtful, scientist were still troubled by the absence of physical evidence that a hard stony object like an asteroid should have left behind.  Then in 2007 a candidate for the long sought impact crater was brought forwards—Lake Cheko, a small, bowl-shaped lake a little more than 4 miles north-northwest of the epicenter.  Magnetic readings indicated a possible meter-sized chunk of rock below the lake’s deepest point that may be a fragment of the colliding body and chemical analysis of the lake silt has supported a creation about the time of the impact.  Scientists from the University of Bologna led by Professor Giuseppe Longo have pressed the case that the long missing impact crater and a fragment may have been found.  Other experts are skeptical.
In 2005 a near earth object identified as 2005 NB56 was observed for a 17 day period as it neared the Earth.  Its exact orbit could not be calculated, but some scientists believe that a large fragment of it in may have brushed the atmosphere in 1908 causing the explosion and then skipping or bouncing back into orbit around the sun.  They believe that the object will again near the earth in 2048 and hope that better calculation of its orbit would be able to confirm it as a candidate.
A couple of proposals have been put forward involving a “natural H-bomb.”  In these scenarios unusually large concentrations of deuterium—heavy hydrogen—in the head of a small comet underwent a nuclear fusion reaction when it entered the atmosphere.  Two or three explanations of how this could have been triggered have been advanced.  Most scientists believe that that the concentration of radioactive isotopes in the blast region to be inconsistent with those expected following a nuclear explosion.
Probably the oddest of all theories seriously advanced was that the earth was actually struck by a small black hole which passed through the planet exiting on the other side.  This one has most scientists shaking their heads in disbelief.  If this were the case there should have been and exit explosion of similar magnitude.  Even though at the expected trajectory, the exit would have occurred somewhere in the North Atlantic, closer than the impact event to the seismic recording stations that collected much of the evidence of the event and would likely have been observed by ships in the region.
A similar proposal suggested a collision with an anti-mater object.  Neither of those explanations takes into account the orbiting dust trails in the atmosphere or the distribution of high-nickel magnetic micro-beads around the impact area.
One scientist has even suggested that there was no collision or impact of any sort, but rather huge eruption and explosion of 10 million tons of natural gas from within Earth’s crust.  Few are taking a bite out of that apple, especially since just as an impact crater has been hard to find, there is no geologic evidence of an outward explosion from the crust.  But with global warming threatening to release vast quantities of frozen methane or natural gas below the Siberian frost line that scenario raises frightening possibilities.
After 120 years tree growth is returning to the devastated area, although many of the logs of the downed trees observed in 1927 lay between new growth.  But at the center,   trees have not returned, although they theoretically should have.  More fodder for scientists and pseudo-scientists alike.
But then so does being struck by an asteroid as scientists believe that a collision with one or more is inevitable and only a matter of time.  Asteroid collisions are the most likely causes for several mass extinctions on Earth—including the one that doomed the dinosaurs. 
Despite all of the other conjectures most scientists keep coming back to that wayward asteroid.
But I am sure as I type that the History Chanel is preparing a “documentary” on the Tunguska Event blaming aliens.  Or maybe they already have….

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