100 years on, mystery shrouds Tunguska impact

Agence France-Presse
A hundred years ago, a gigantic explosion ripped open the dawn sky above the swampy taiga forest of western Siberia, leaving a scientific riddle that endures to this day.

A dazzling light pierced the heavens, preceding a shock wave with the power of a thousand atomic bombs which flattened 80 million trees over a swathe of more than 2,000 km2. Local Evenki nomads recounted how the blast tossed homes and animals into the air. In Irkutsk, 1,500 km away, seismic sensors registered what was initially deemed to be an earthquake.
What caused the so-called Tunguska Event, named after the Podkamennaya Tunguska river near where it happened, has spawned at least a half a dozen theories.

Finger of blame

The biggest finger of blame points at a rogue rock whose destiny – after travelling in space for millions of years – was to intersect with Earth at exactly 7:17 am on 30 June 1908.
Even the most ardent defenders of the sudden impact theory acknowledge that there are many gaps, however. They strive to find answers, believing this will strengthen defences against future Tunguska-type threats, which experts say occur at a frequency ranging from one-in-200 years to one-in-1,000 years.
"Imagine an unspotted asteroid laying waste to a significant chunk of land and imagine if that area, unlike Tunguska and a surprising amount of the globe today, were populated," the British science journal Nature commented last week.
If a rock was the culprit, the choices lie between an asteroid – the rubble that can be jostled out of its orbital belt between Mars and Jupiter and set on collision course with Earth – and a comet, one of the "icy dirtballs" of frozen, primeval material that loop around the Solar System.

Why no fragments?

Comets move at far greater speeds than asteroids, which means they release more kinetic energy pound-for-pound upon impact. A small comet would deliver the same punch as a larger asteroid. But no fragments of the Tunguska villain have ever been found, despite many searches.
Finding a piece is important, for it will boost our knowledge about the degrees of risk from dangerous Near Earth Objects (NEOs), said Italian researchers Luca Gasperini, Enrico Bonatti and Giuseppe Longo based at the Marine Science Institute in Bologna.
When a new asteroid is detected, its orbit can be plotted for scores of years in the future. Comets are far less numerous than asteroids but are rather more worrying, as they are a largely unknown entity.
Most comets have yet to be spotted because they take decades or even hundreds of years to go around the Sun and pass our home. As a result, any comet on a collision course with Earth could quite literally come out of the dark, leaving us negligible time to respond.

Unique or regular?

"If the Tunguska event was in fact caused by a comet, it would be a unique occurrence rather than an important case study of a known class of phenomena," Gasperini's team wrote in last month's issue of Scientific American magazine. "On the other hand, if an asteroid did explode in the Siberian skies that June morning, why has no one yet found fragments?"
NEO experts are likewise unsure about the size of the object. Estimates, based on the scale of ground destruction, range from three to seventy metres across.
All agree that the object, heated by friction with atmospheric molecules, exploded far above ground – up to 10 km away. But there is fierce debate as to whether any debris hit the ground.
This information is also important. When the next Tunguska NEO looms, Earth's guardians will have to choose whether to try to deflect it or blow it up in space, with the risk that objects of a certain size may survive the fiery passage through the atmosphere and hit the planet.
The Italian trio believe the answers lie in a curiously shaped oval lake, called Lake Cheko, located about 10 km from ground zero. Computer models, they said, suggest it is the impact crater from a metre-sized fragment that survived the explosion.
They plan a return expedition to Lake Cheko in the hope of recovering a dense object of this size, buried 10 m below the lake's cone-shaped floor. Something fitting that description has already been detected with sonar waves.

Rival theory

But what if neither comet nor asteroid were to blame? A rival theory is given an airing in New Scientist magazine.
Lake Cheko does not have the typical round shape of an impact crater, and no extraterrestrial material has been found, which means "there's got to be a terrestrial explanation," Wolfgang Kundt, a physicist at Germany's Bonn University told the British weekly.
He believes the Tunguska Event was caused by a massive escape of 10 million tonnes of methane-rich gas deep within Earth's crust. Evidence of a similar apocalyptic release can be found on the Blake Ridge on the seabed off Norway, a "pockmark" of 700 km2, Kundt said.

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