quinta-feira, 27 de junho de 2013

Mystery solved: meteorite caused Tunguska devastation

A century after, scientists have revealed that the 1908 Tunguska devastation was caused by a meteorite by identifying 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.

By Simon Redfern, University of Cambridge

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, 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.

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.

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 datademonstrates that the energy of the impact was equivalent to a 460 kiloton (TNT) bomb, which is about 40 times the Hiroshima blast.

Simon Redfern does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

Keywords: Tunguska event, Chelyabinsk meteor, lonsdaleite, National Academy of Sciences of Ukraine, Arizona impact, The Conversation

Source: thehindu.com

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