Tuesday, June 10, 2008

Tunguska, A Century Later

Asteroid or comet blamed for Siberian blast of 1908
The Tunguska blast shook Siberia in 1908, but on-site investigations were delayed for two decades. One of the first photos showed a large area of flattened trees.

Early on the morning of June 30, 1908, a massive explosion shook central Siberia. Witnesses told of a fireball that streaked in from the southeast and then detonated in the sky above the desolate, forested region. At the nearest trading post, about 70 kilometers away from the blast, people were reportedly knocked from their feet. Seismic instruments in the area registered ground motions equivalent to those of a magnitude-5 earthquake.

Effects of the event—often called the Tunguska blast, after a major river running through the area—weren’t restricted to Siberia. Sensitive barometers in England detected an atmospheric shock wave as it raced westward and then detected it again after it traveled around the world. High-altitude clouds that formed over the region after the event were so lofty that they caught light from beyond the horizon, illuminating the sky so much that people at locales in Europe and Asia could read newspapers outdoors at midnight.

A number of factors—including the site’s remote location, World War I and the Russian Revolution—prevented scientists from mounting an expedition to the blast zone for almost two decades, says physicist Giuseppe Longo of the University of Bologna in Italy. When researchers eventually reached the region, they found that a 2,150-square-kilometer patch of forest had been flattened, with most of the 80 million trees lying in a radial pattern. What the researchers didn’t find, however, was an obvious crater.

A century later, scientists are still debating the cause of the Tunguska blast. Through the years, Longo notes, a variety of scenarios have been proposed, many of them involving the explosion of an unusual extraterrestrial object—everything from a small black hole or a chunk of antimatter to a UFO. Most researchers, however, now pin the blame on the mid-air explosion of a small comet or asteroid, which typically can’t stand up to the pummeling received while blazing through the atmosphere.

The damage in Siberia suggests that the Tunguska detonation happened at an altitude of between 6 and 8 kilometers and released the energy of about 15 megatons of TNT, about a thousand times more than the bomb that destroyed Hiroshima.

Data gathered by military satellites—including those designed to detect clandestine nuclear explosions—suggest that tiny versions of the Tunguska blast occur rather frequently, says Philip A. Bland, a planetary scientist at Imperial College London. The largest airburst detected during the 1990s measured only a few tens of kilotons, the energy release expected from the explosion of an asteroid measuring about 7 or 8 meters across. Impacts of objects measuring at least 1 meter in diameter occur, on average, about once a week, the data suggest. Tunguska-sized airbursts would be expected to happen about once every 500 years, says Bland.

Previous studies have estimated that the Tunguska asteroid was between 50 and 80 meters in diameter, says Mark Boslough, a physicist at Sandia National Laboratories in Albuquerque, N.M. However, new supercomputer simulations by Boslough and his colleagues hint that a much smaller object could have produced the damage. New models account for the downward momentum of the air compressed beneath the incoming object, a component of the process “that previously had been thrown away,” says Boslough.

So the Tunguska asteroid may have been only 30 to 50 meters across, he notes. Simulations suggest that it entered the atmosphere traveling about 15 kilometers per second at an angle about 35 degrees above the horizon. The shock wave produced by the airburst could have slammed into the ground at 180 kilometers per hour, a gust with the wind speed of a category-3 hurricane, the team reports in an upcoming issue of the International Journal of Impact Engineering.

Late last year, Longo and his colleagues reported that Lake Cheko, a 400-meter–wide lake about eight kilometers northwest of the Tunguska blast’s epicenter, could be the long-sought crater produced by a chunk of asteroid that actually reached the ground. The lake is about 50 meters deep, has a cone-shaped bottom unlike other lakes in the region and—possibly most important—lies directly along the estimated path of the fireball. Sonar studies reveal a buried object or a densely compacted layer of sediment about 10 meters below the center of the lake bottom, the researchers reported in the August 2007 Terra Nova.

CELESTIAL CAUSE? Supercomputer simulations show how an asteroid would approach and explode with the force and extent known for the Tunguska event.
Sandia National Lab

But other factors suggest that Lake Cheko isn’t a water-filled impact crater, says Gareth Collins of Imperial College London. For one thing, he notes, a hole the size of this lake typically would be one in a series of holes excavated by pieces of the disintegrating object, whereas Lake Cheko apparently has no companions. And the area around the lake isn’t covered with a layer of material that would have been thrown out of a crater during the impact. Also, pictures of the lake from an aerial survey in 1938 show mature trees (more than 30 years old) on the lakeshore—a sure sign that this body of water has a more benign provenance, Collins and colleagues write in the April Terra Nova.

The lack of an impact crater, along with the dearth of geochemical anomalies in rock in the region, has spurred some scientists to seek an alternate explanation for the blast. One favorite down-to-Earth idea points to the modern-day formation of a kimberlite deposit, an eruption that brings diamonds to Earth’s surface (SN: 6/30/07, p. 412). Such an eruption could have injected about 10 million tons of methane into the atmosphere, a plume that if detonated would have released a forest-flattening burst of energy.

Scientists will attend conferences in Moscow at the end of June to commemorate the blast and discuss the latest findings. Those proposing an extraterrestrial cause will be meeting at the Russian Academy of Sciences. Those who favor an Earth-based origin of the blast will gather across town, at the Polytechnical Museum.

Back story

Explanations for the Tunguska event have spanned science and fiction

1925-A Russian scientist says the explosion’s recorded seismic and air waves suggest a meteorite as the culprit.

1934-A British scientist and a Russian scientist say the blast must have been an in-air comet explosion.

1941-An American researcher claims an antimatter meteoroid was annihilated when it encountered matter.

1946-Russian sci-fi writer Alexander Kazantsev tells the tale of Tunguska as a UFO that vaporized.

1973-Two theorists suggest a black hole as small as a comma passed through Earth and exited via the North Atlantic.

2001-Ideas go down: The blast perhaps was a kimberlite eruption, which lifts diamonds and methane from Earth’s depths.


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