The explosion that occurred over Russia in 1908 is suspected to have been a comet or asteroid, and this impact could have also formed Lake Cheko.
At 0:14 UT near the Podkamennaya Tunguska riverbed in what is now Krasnoyarsk Krai, Russia, witnesses reported seeing a column of light in the sky shortly followed by a shockwave which would have measured 5.0 on the Richter scale. Windows were destroyed in structures several hundred miles away. Several nights following the June 30, 1908 event, Asian and European skies were aglow with what is speculated to have been ice particles lingering after a comet exploded in the atmosphere, and a decrease in atmospheric transparency caused by dust particles was present for several months after.
Producing atmospheric pressure changes that could be detected in England, the 10 to 15 megaton explosion occurred between 6 and 10 miles above the earth’s surface. That amount of energy released is approximately equivalent to 1,000 times that of the bomb released on Hiroshima, Japan. The object is generally theorised to have been between approximately 6 and 19 miles across.
Evidence Supporting Meteorite Impact
The first recorded expedition to visit the site was made by Leonid Kulik in 1921 as part of a survey. He examined local reports of the event and surmised that the event was caused by a meteorite. Kulik maintained active interest in the site for the next two decades: in 1927 he discovered the 830 square mile zone directly damaged by the blast, and in 1938, he arranged for an aerial photography run over the site. The negatives resulting from the aerial shoot were destroyed by Yevgeny Krinov in 1975.
It’s possible that expeditions conducted before Kulik’s journey in 1921 were lost to history, having been misplaced or destroyed during such historical events as World War I, the Russian Revolution of 1917 and the Russian Civil War.
Further expeditions conducted in the 1950s and 1960s revealed silicate and magnesium spheres of a microscopic size embedded within dirt samples and resin samples from trees in the area. The proportions of nickel versus iron within these spheres suggests a comparable composition to those found in meteorites. In addition to their composition, their distribution patterns are consistent with what would be expected from an explosion in the air. Another study of peat bogs in the area confirmed that they contained anomalous isotopic signatures during times corresponding to the 1908 event.
Disputes Between Astroid or Comet Origin
One of the most notable proponents of the comet theory is astronomer L’ubor Kresák, who proposed in 1978 that the Tunguska event was caused by a fragment of Comet Encke. The event occurred both during a peak period in a shower caused by this comet and at a calculated trajectory congruent with a fragment having broken away and fallen into the atmosphere. Despite assertions that a comet would have disintegrated in the upper levels of the atmosphere, advocates of this theory suggest that it was an extinct comet—one that had burned up all of its icy fuel and remained a dark, small hunk of rubble or rock. Comet theorists also point to the lack of a crater as evidence against the object having been an asteroid. In support of this theory, research published in the Geophysical Research Letters suggests that noctilucent clouds present after the impact support the theory that the impact was caused by a comet, the clouds being a byproduct of large amounts of vaporised water in the atmosphere.
Alternatively, in 1983 this theory was criticised by Zdenek Sekanina who first pointed out that a comet would have broken apart in the higher atmosphere. Claiming the Tunguska object to have been rocky and most likely of asteroidal origin, his theory was further propagated in the 1990s when tested resin samples returned material levels found commonly in asteroids and uncommonly in comets, and in 2001 when Farinella et al. released information suggesting the asteroid belt as the origin of the meteorite.
The Possible Origin of Lake Cheko
Italian scientists from the University of Bologna have also put forth the theory that Lake Cheko, five miles north-northwest of the impact site, is a direct result of a 1-metre wide fragment of rock having survived the air blast and striking the ground. Study and testing conducted at the lake reveal a conical lakebed consistent with the crater of an impact; at the deepest point of the lakebed lay a 1-metre wide chunk of rock possibly responsible for its formation. Estimated to be around 100 years old, its long axis points directly at the hypocentre of the impact, 4.3 miles away.