[meteorite-list] Crater From 1908 Russian Space Impact Found, Team Says

From: Mike Groetz <mpg444_at_meteoritecentral.com>
Date: Thu, 8 Nov 2007 16:32:08 -0800 (PST)
Message-ID: <355365.96677.qm_at_web32908.mail.mud.yahoo.com>

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

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
(see map).

On June 30, 1908, a ball of fire exploded about 6
miles (10 kilometers) above the ground in the sparsely
populated region, scientists say. The blast released
15 megatons of energy?about a thousand times that of
the atomic bomb dropped on Hiroshima?and flattened 770
square miles (2,000 square kilometers) of forest.

Since then many teams of scientists have combed the
site, but none was able to find any fragments of an
object, like a rocky asteroid or a comet, that might
have caused the event.

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.

"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).

It also lacks the rim of debris usually found around
typical impact craters, such as the Meteor Crater in
Arizona.

Gasperini's team says that the basin's unusual shape
is the result of a fragment thrown from the Tunguska
explosion that plowed into the ground, leaving a long,
trenchlike 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, he added, 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.

In a previous expedition, Russian scientists studied
Lake Cheko and concluded that it had formed before
1908, indicating that it was not formed by the
Tunguska event.

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.

Asteroid or Comet?

William Hartmann, senior scientist of the Planetary
Science Institute in Tucson, Arizona, said 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," he said. "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?"

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, Hartmann
said.

"Our crater hypothesis is consistent with both
possibilities," Gasperini said.

"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."

Gasperini and his colleagues are planning to go back
to Siberia next year to search for more, and perhaps
more conclusive, clues to the century-old puzzle.

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

 


 



__________________________________________________
Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around
http://mail.yahoo.com
Received on Thu 08 Nov 2007 07:32:08 PM PST