[meteorite-list] More on Tunguska and Lake Cheko

From: Jerry <grf2_at_meteoritecentral.com>
Date: Thu, 08 Nov 2007 20:36:52 -0500
Message-ID: <BF03566D13EF4DFB9CB24390789B6AF7_at_Notebook>

Thanks, always an interesting subject.
Jerry Flaherty
----- Original Message -----
From: "Darren Garrison" <cynapse at charter.net>
To: <meteorite-list at meteoritecentral.com>
Sent: Thursday, November 08, 2007 11:58 AM
Subject: [meteorite-list] More on Tunguska and Lake Cheko


http://www.inform.kz/showarticle.php?lang=eng&id=157070

08.11.2007 / 09:32 Crater from 1908 Russian space impact found, team says
NEW YORK. November 8. KAZINFORM. 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.

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, Kazinform quotes National Geographic
News.

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.
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Received on Thu 08 Nov 2007 08:36:52 PM PST