100 Years Since Tunguska Blast

3 07 2008

Photograph from the Soviet Academy of Science 1927 expedition led by Leonid Kulik.The 30th of June saw the 100th anniversary of the Tunguska Event in Russia.

For those of you not familiar with this event it was a massive explosion near the Stony Tunguska River in Siberia that knocked down 80 million trees in a 2,000 square kilometer area and it triggered seismographs around the world.

Over the years many wacky explanations have been put forward to explain it including a small black hole passing through the Earth, antimatter striking the Earth, and of course a UFO that crashed. Imagine that, navigating your way for hundreds, possibly thousands of light years, only to make some error or have a catastrophic mechanical failure right at the last moment…bugger!

The most likely explanation for this event is still something that is very cool and a phenomenon that one day may have a massive impact (mind the pun) on human civilization. In this case a meteor a few tens of metres across exploded in the atmosphere 6 to 10 km above the Earth’s surface with an explosion of about 15 megatons. Speculation about what it could be was driven by the a lack of understanding about meteors in the early part of last century and an apparent lack of a crater that would have provided an immediate indication that it was an object that had fallen from the heavens.

So then, what evidence is there to support that it was a meteor? Well to summarise:

  • Witnesses observed a bright object moving across the sky prior to the explosion, one witness said they watched it descend for 10 minutes.
  • Meteorites usually contain large amounts of nickel and studies of the trees in the vicinity of the explosion were found large amounts of nickel relative to iron. Other studies found that they also had other unusual ratios of metals when compared with samples taken from outside of the blast area that were consistent with meteorites.
  • When the 1908 layers in the bogs were examined in the blast area they were found to contain a number of elements whose isotopic signature did not match the normal decay of elements found elsewhere on Earth.
  • This layer was also found to contain the rare element iridium and similarities were noted with the K-T boundary at the dinosaurs extinction.
  • The ongoing monitoring of meteors entering Earth’s atmosphere has determined that airbursts are a common occurrence.

So in all likelihood rather than it being little green men who have fallen asleep at the wheel after a long trip, a meteor is a more likely explanation. Tunguska is another excellent example of a mystery that with time and careful study it can be explained.




One response

10 07 2008

There was an article about this in last week’s New Scientist. The upshot of the article was that there were two main theories:
1. A bolide of some kind (asteroid or comet, etc)
2. An escape of high-pressure gas from deep within the earth.

According to the article, where it stands at the moment there isn’t any extraterrestrial evidence for a bolide and they have found no kimberlite pipes to indicate an ‘Earth-burp’.

They don’t mention the paper you link to about iridium in the area though and focus more on the debate around the origins of Lake Cheko.

Either way, the explanation of little green men is fanciful. It would be great if we were visited by aliens but we shouldn’t let our fantasies get the better of ourselves.

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