Tunguska Blast Crater Found
Lake Cheko Published:12:44 pm. -1-20-11

Crater From 1908 Russian Space Impact Found, Team Says Maria Cristina Valsecchi in Rome, Italy for National Geographic News

November 7, 2007

Almost a century after a mysterious explosion in Russia flattened a huge swath of Siberian forest, scientists have found what they believe is a crater made by the cosmic object that made the blast.

The crater was discovered under a lake near the Podkamennaya Tunguska River in western Siberia, where the cataclysm, known as the Tunguska event, took place.

In their new study, a team of Italian scientists used acoustic imagery to investigate the bottom of Lake Cheko, about five miles (eight kilometers) north of the explosion's suspected epicenter.

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

"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.

A "Soft Crash"

The basin of Lake Cheko is not circular, deep, and steep like a typical impact crater, the scientists say.

Instead it's elongated and shallow, about 1,640 feet (500 meters) long with a maximum depth of only 165 feet (50 meters).

The team had measured sediments on the bottom of the lake and determined that the deposits were accumulating there at about 0.4 inch (1 centimeter) a year. This suggested that Lake Cheko was several centuries old.

But Gasperini's team argues that the older deposits found by the Russians were already there when the explosion took place.

"We found evidence that only the topmost, one-meter-deep [three-foot-deep] layer of debris actually came from the inflowing river," Gasperini said.

"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."

The team's findings are based on a 1999 expedition to Tunguska and appeared in the August issue of the journal Terra Nova.

The Tunguska Blast The only major meteoroid impact recent enough to have first-hand accounts. Published as a NASA feature in June, 2008.

Tunguska Blast Site One hundred years ago an asteroid of about 220 million pounds entered Earth's atmosphere above Siberia at a speed of about 33,500 miles per hour. It never hit the ground. Instead it heated the air around it to about 44,500 degrees Fahrenheit and exploded at an elevation of about 28,000 feet. The energy produced was nearly 200 times more powerful than the Hiroshima bomb.

Some of the science behind this event is explained below along with eyewitness information and observations of the blast. Although large impacts are quite rare one of this magnitude is thought to occur about once every three hundred years.

At around 7:17 on the morning of June 30, 1908, a man based at the trading post at Vanavara in Siberia is sitting on his front porch. In a moment, 40 miles from the center of an immense blast of unknown origin, he will be hurled from his chair and the heat will be so intense he will feel as though his shirt is on fire.

The man at the trading post, and others in a largely uninhabited region of Siberia, near the Podkamennaya Tunguska River, are to be accidental eyewitnesses to cosmological history.

Don Yeomans NASA "If you want to start a conversation with anyone in the asteroid business all you have to say is Tunguska," said Don Yeomans, manager of the Near-Earth Object Office at NASA's Jet Propulsion Laboratory. "It is the only entry of a large meteoroid we have in the modern era with first-hand accounts."

While the impact occurred in '08, the first scientific expedition to the area would have to wait for 19 years.

In 1921, Leonid Kulik, the chief curator for the meteorite collection of the St. Petersburg museum led an expedition to Tunguska. But the harsh conditions of the Siberian outback thwarted his team's attempt to reach the area of the blast.

In 1927, a new expedition, again lead by Kulik, reached its goal.

"At first, the locals were reluctant to tell Kulik about the event," said Yeomans. "They believed the blast was a visitation by the god Ogdy, who had cursed the area by smashing trees and killing animals."

While testimonials may have at first been difficult to obtain, there was plenty of evidence lying around.

Eight hundred square miles of remote forest had been ripped asunder. Eighty million trees were on their sides, lying in a radial pattern.

"Those trees acted as markers, pointing directly away from the blast's epicenter," said Yeomans. "Later, when the team arrived at ground zero, they found the trees there standing upright -- but their limbs and bark had been stripped away. They looked like a forest of telephone poles."

Tunguska Blast Site Such debranching requires fast moving shock waves that break off a tree's branches before the branches can transfer the impact momentum to the tree's stem.

Thirty seven years after the Tunguska blast, branchless trees would be found at the site of another massive explosion -- Hiroshima, Japan.

Kulik's expeditions (he traveled to Tunguska on three separate occasions) did finally get some of the locals to talk. One was the man based at the Vanara trading post who witnessed the heat blast as he was launched a few yards.

His account:

Suddenly in the north sky… the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire... At that moment there was a bang in the sky and a mighty crash... The crash was followed by a noise like stones falling from the sky, or of guns firing. The earth trembled.

The massive explosion packed a wallop. The resulting seismic shockwave registered with sensitive barometers as far away as England. Dense clouds formed over the region at high altitudes which reflected sunlight from beyond the horizon.

Night skies glowed, and reports came in that people who lived as far away as Asia could read newspapers outdoors as late as midnight.

Hundreds of reindeer, the livelihood of local herders, were killed, but there was no direct evidence that any person perished in the blast.

"A century later some still debate the cause and come up with different scenarios that could have caused the explosion," said Yeomans. "But the generally agreed upon theory is that on the morning of June 30, 1908, a large space rock, about 120 feet across, entered the atmosphere of Siberia and then detonated in the sky."

It is estimated the asteroid entered Earth's atmosphere traveling at a speed of about 33,500 miles per hour. During its quick plunge, the 220-million-pound space rock heated the air surrounding it to 44,500 degrees Fahrenheit.

At 7:17 a.m. (local Siberia time), at a height ofabout 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and annihilate itself, producing a fireball and releasing energy equivalent to about 185 Hiroshima bombs.

"That is why there is no impact crater," said Yeomans. "The great majority of the asteroid is consumed in the explosion."

Yeomans and his colleagues at JPL's Near-Earth Object Office are tasked with plotting the orbits of present-day comets and asteroids that cross Earth's path, and could be potentially hazardous to our planet.

Yeomans estimates that, on average, a Tunguska-sized asteroid will enter Earth's atmosphere once every 300 years. On this 100th anniversary of the Tunguska event, does that mean we have 200 years of largely meteor-free skies?

"Not necessarily," said Yeomans. "The 300 years between Tunguska-sized events is an average based on our best science. I think about Tunguska all the time from a scientific point of view, but the thought of a another Tunguska does not keep me up at night."

Tunguska Blast Site Today

Tunguska Blast Site

Leonid Kulik and His Expedition

Tunguska Blast Site

Source & References: Maria Cristina Valsecchi

http://news.nationalgeographic.com/news/2007/11/071107-russia-crater.html

http://geology.com/nasa/tunguska-blast.shtml

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