The hypothesis regarding Lake Cheko

Nov 8, 2007 09:45 GMT  ·  By

On June 30, 1908, the biggest space impact that Earth suffered in modern times, known as the Tunguska event, took place in a remote Siberian area, destroying more than 2,000 sq km (770 square mi) of forest near the Tunguska River (central Siberia). The ball of fire that could have been a comet or asteroid, blasted about 6 mi (10 km) off the ground in the atmosphere with a power similar to 1,000 Hiroshima bomb explosions (20 millions tones of TNT), so brightly and powerfully that it was seen even on the London sky.

For a century, many teams have been attempting to find the culprit object, combing the site, but none could discover any fragments of an object that could have triggered the event. Now an Italian team, publishing its results which appeared in the August issue of the journal Terra Nova, used acoustic imagery pointing that the mysterious object could be found on the bottom of Lake Cheko, about 5 mi (8 km) north of the explosion's epicenter.

"When our expedition [was at] Tunguska, we didn't have a clue that Lake Cheko might fill a crater," said lead author Luca Gasperini, a geologist with the Marine Science Institute in Bologna.

"We searched its bottom looking for extraterrestrial particles trapped in the mud. We mapped the basin and took samples. As we examined the data, we couldn't believe what they were suggesting. The funnel-like shape of the basin and samples from its sedimentary deposits suggest that the lake fills an impact crater," Gasperini said.

Lake Cheko does not match the profile of an 'impact' lake: it is neither circular nor deep, like normal impact lakes and it also lacks the debris like in the case of the Meteor Crater in Arizona. This Siberian lake is rather elongated and shallow, 1,640 ft (500 m) long with a maximum depth of just 165 ft (50 m).

"The Cheko feature was "anomalously" shallow and lacked the round shape of most craters - being more elliptical in its form. Elliptical craters only occur if the impactor's angle of entry is less than about 10 degrees. We know from modelling of the Tunguska event that the angle of entry must have been steeper than that," Dr Gareth Collins, a research associate at Imperial College London, UK, told BBC News.

The Italian team explains that this shape is so odd due to a space body fragment plowing into the ground, leaving behind this type of depression. "We suggest that a 10-meter-wide [33-foot-wide] fragment of the object escaped the explosion and kept going in the same direction. It was relatively slow, about 1 kilometer a second [0.6 mile a second]," Gasperini said.

"The lake is located along the most probable track of the cosmic body, which likely made a "soft crash" in the marshy terrain. It splashed on the soft, swampy soil and melted the underlying permafrost layer, releasing CO2 [carbon dioxide], water vapor, and methane that broadened the hole, hence the shape and size of the basin, unusual for an impact crater. Our hypothesis is the only one that accounts for the funnel-like morphology of Lake Cheko's bottom," he added.

A previous Russian research found that the lake was formed before 1908, not being the result of the Tunguska event. The sediments on the bottom of the lake stored at the rhythm of 0.4 inch (1 centimeter) annually, pointed that Lake Cheko was several centuries old.

The Italian team says that the older layers investigated by the Russians were already there before the event.

"We found evidence that only the topmost, one-meter-deep [three-foot-deep] layer of debris actually came from the inflowing river. [The] deeper sediments are deposits that predate 1908. They were the target over which the impact took place, so Lake Cheko is only one century old." said Gasperini.

The lake bottom's relief has a funnel-like shape not encountered in neighboring lakes and under 10m, the lake bed shows a disconnection that could be either compacted lake sediments or a fragment of space rock.

"We have no positive proof this is an impact crater, but we were able to exclude some other hypotheses, and this led us to our conclusion," lead researcher Professor Giuseppe Longo, told BBC News.

There is another trait of the impact craters that cannot be found on Lake Cheko - a "flap" surrounding the crater rim made of upside-down material thrown at short distance by the hit.

"If pieces of the space rock had survived the airburst, they would have been too small and traveling too slowly to have generated a crater the size of Lake Cheko. An impact would also have felled trees all around the crater, yet there appeared to be trees older than 100 years still standing around Lake Cheko today", said Collins.

Computer models showed that centimeter-sized rocks could have been tossed hundreds of kilometers away from Tunguska.

The Italian researchers say that the uncommon structure could be explained by a slower speed of the cosmic rock and attenuated impact on the swampy Siberian taiga and the crater would have been enlarged by the water and gas expelling from the ground.

"This could also account for the limited damage to the surrounding area and the absence of a rim of upturned ejecta. If formed during the impact, [the rim] would have been rapidly obliterated by collapse and gravity-failures during the subsequent degassing phase," the authors wrote.

"The new findings are compelling but do not address all of the lingering questions about the event. It's an exciting result that might shed new light on the Tunguska explosion. Certainly it warrants new studies of the area. But it raises a question in my mind: If one large fragment hit the ground, we would normally expect thousands of smaller fragments also to hit the ground along the path, and many searches have failed to find such meteorite fragments. So, why no smaller pieces?" said William Hartmann, senior scientist of the Planetary Science Institute in Tucson, Arizona.

"Finding fragments from the explosion is considered key to determining what kind of object made the impact. An asteroid would probably leave some remains, while a comet might be annihilated in the blast," said Hartmann.

"Our crater hypothesis is consistent with both possibilities. If the body was an asteroid, a surviving fragment may be buried beneath the lake. If it was a comet, its chemical signature should be found in the deepest layers of sediments." said Gasperini.

The last Italian expedition in the area took place in 1999 and the next one could take place in 2008, aiming at drilling probes up to 10m below the lake bed to the anomaly investigated by the geophysics, to assess the nature of that rock.

"We want to dig deeply in the bottom of the lake to definitively test our hypothesis and try to solve the Tunguska mystery," said Gasperini.

Photo Gallery (3 Images)

The forest in the nearby area after the explosion
The Tunguska blastLake Cheko
Open gallery