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Mystery solved: meteorite caused Tunguska devastation

On the morning of June 30 in 1908, a gigantic fireball devastated hundreds of square kilometres of uninhabited Siberian forest around the Tunguska river. The first scientists to investigate the impact…

Vast areas were flattened by a meteorite in Tunguska in 1908. Leonid Kulik

On the morning of June 30 in 1908, a gigantic fireball devastated hundreds of square kilometres of uninhabited Siberian forest around the Tunguska river. The first scientists to investigate the impact site expected to find a meteorite, but they found nothing.

Because no traces of a meteorite were found, it many scientists concluded that the culprit was a comet. Comets, which are essentially muddy ice balls, could cause such a devastation and leave no trace.

But now, 105 years later, scientists have revealed that the Tunguska devastation was indeed caused by a meteorite. A group of Ukrainian, German, and American scientists have identified its microscopic remains. Why it took them so many years makes for a fascinating tale about the limits of science and how we are pushing them.

Big ball of fire

Eyewitness reports of the Tunguska event help paint a partial picture. As the fireball streaked across the sky, a blast of heat scorched everything in its wake, to be followed by a shock wave that threw people off their feet and stripped leaves and branches from trees, laying a large forest flat. Photos reveal the extent and force of the impact, showing trees that look like bare telegraph poles, all pointing away from the impact site.

Forest at Tunguska after the 1908 impact.

The inability to find any meteorite, however, led to a century of speculation on the origins of the blast. The Tunguska event has spawned a wealth of science fiction that has fed outrageous theories. But the main question has remained: what was it?

An icy comet would evaporate on impact, which could explain the lack of any observable evidence. But a study in the journal Planetary and Space Science provides, for the first time, evidence that the impact was not caused by a comet. Researchers collected microscopic fragments recovered from a layer of partially decayed vegetation (peat) that dates from that extraordinary summer.

Fallen trees radiate out from the impact focus, like matchsticks.

Victor Kvasnytsya from the National Academy of Sciences of Ukraine and his colleagues used the latest imaging and spectroscopy techniques to identify aggregates of carbon minerals—diamond, lonsdaleite, and graphite. Lonsdaleite in particular is known to form when carbon-rich material is suddenly exposed to a shock wave created by an explosion, such as that of meteorite hitting Earth. The lonsdaleite fragments contain even smaller inclusions of iron sulphides and iron-nickel alloys, troilite and taenite, which are characteristic minerals found in space-based objects such as meteorites. The precise combination of minerals in these fragments point to a meteorite source. It is near-identical to similar minerals found in an Arizona impact.

The samples point to one thing: the Tunguska impact is the largest meteorite impact in recorded history. US researchers have estimated that the Tunguska blast could have been as much as the equivalent of a five megaton TNT explosion—hundreds of times more powerful than the Hiroshima blast. The meteorite tore apart as it entered the atmosphere at an angle, so that little of it reached the ground intact. That is why all remains are such small specks that have been fossilised in the Siberian peat.

2013 meteorite impact

We can compare the Tunguska event with the fireball seen during the impact of the Chelyabinsk meteor earlier this year. Although much less powerful than Tunguska, the Chelyabinsk event was similar. A low-angle approach broke up the body, leaving fragments that were found over the vast expanse of Eurasia. More than 1,000 people were injured, some drawn to windows by the flash of the fireball and then hit by the shock wave that followed.

The Tunguska devastation that was not investigated for 19 years, partly because of lack of resources. In contrast, the Chelyabinsk meteorite attracted immediate attention. Dashboard cameras captured the trajectory and brightness of the fireball, while CCTV networks provided fixed reference points. The US space agency NASA has now been able to identify the origins of the meteorite.

The low-frequency rumble of the Chelyabinsk event travelled twice around the globe. The data demonstrates that the energy of the impact was equivalent to a 460 kiloton (TNT) bomb, which is about 40 times the Hiroshima blast.

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3 Comments sorted by

  1. George Michaelson

    Person

    "That is why all remains are such small specks that have been fossilised in the Siberian peat."

    Um. its highly specific inorganic minerals, formed by intense pressure and heat. Fossils are sedimentary deposits formed very very slowly replacing biological material. (I am not a geologist. somebody can correct me)

    A great metaphor for another timer, but perhaps not a good fit for this purpose.

    "Thats why all remains are such small specks that have been preserved in the Siberian peat, but eluded discovery until now"

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    1. Simon Redfern

      Professor in Earth Sciences at University of Cambridge

      In reply to George Michaelson

      I'm not one for semantics, George. But it is interesting that in the early days of Earth Sciences the word "fossil" was used to describe anything that had been dug out of the ground. There are other physical processes captured in time within sediment that are termed fossils, for example fossil fulgurites. But it's an interesting point you make, because in this case the meteorite mineral fragments are inorganic and the (biological organic) peat form which they are extracted form the current "sediment", which is stratified. Maybe this should be called an "anti-fossil"?!

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    2. George Michaelson

      Person

      In reply to Simon Redfern

      I live and learn! Had no idea fossil had a wider meaning which has narrowed like that.

      Did the stratification of the peat aides in determining the timing of the deposit? I do hope so!

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