In late
June of 1908, a fireball exploded above the remote Russian forests of Tunguska,
Siberia, flattening more than 800 square miles of trees. Researchers think a
meteor was responsible for the devastation, but neither its fragments nor any
impact craters have been discovered.
Astronomers
have been left to guess whether the object was an asteroid or a comet, and
figuring out what it was would allow better modeling of potential future
calamities.
Italian
researchers now think they've found a smoking gun: The 164-foot-deep Lake Cheko, located just 5 miles northwest of the epicenter of destruction.
"When we looked at the bottom of the
lake, we measured seismic waves reflecting off of something," said Giuseppe
Longo, a physicist at the University of Bologna in Italy and co-author of the
study. "Nobody has found this before. We can only explain that and the shape of the lake as a
low-velocity impact crater."
Should the
team turn up conclusive evidence of an asteroid or comet on a later expedition,
when they obtain a deeper core sample beneath the lake, remaining mysteries
surrounding the Tunguska event may be solved.
The
findings are detailed in this month's online version of the journal Terra
Nova.
Submerged
evidence
During a
1999 expedition, Longo's team didn't plan to investigate Lake Cheko as an impact crater, but rather to look for meteoroid
dust in its submerged sediments. While sonar-scanning the lake's
topography, they were struck by its cone-like features.
"Expeditions
in the 1960s concluded the lake was not an impact crater, but their
technologies were limited," Longo said. With the advent of better sonar and
computer technologies, he explained, the lake took shape.
Going a
step further, Longo's team dove to the bottom and took 6-foot core samples,
revealing fresh mud-like sediment on top of "chaotic deposits" beneath. Still,
Longo explained the samples are inconclusive of a meteorite impact.
"To really
find out if this is an
impact crater," Longo said, "we need a core sample 10 meters (33
feet) into the bottom" in order to investigate a spot where the team detected a
"reflecting" anomaly with their seismic instruments. They think this could be
where the ground was compacted by an impact or where part of the meteorite
itself lies: The object, if found, could be more than 30 feet in diameter and
weigh almost 1,700 tons — the weight of about 42 fully-loaded semi-trailers.
Caution
for now
From a UFO
crash to a wandering black hole, wild (and wildly unsupported) explanations
for the Tunguska event have been proposed. Alan Harris, a planetary scientist
at the Space Science Institute in Boulder, Colorado, said the proposal by
Longo's team isn't one of them.
"I was
impressed by their work and I don't think it's something you can wave off,"
said Harris, who was not involved in the research.
Longo and
his team "are among the
recognized authorities on Tunguska" in the world, Harris told SPACE.com. "It would be thrilling to dig up
chunks of the meteor body, if they can manage to. It would lay the question to
rest whether or not Tunguska was a comet or asteroid."
Some
researchers, however, are less confident in the team's conclusions.
"We know
from the entry physics that the largest and most energetic objects penetrate
deepest," said David
Morrison, an astronomer with NASA's Ames Research Center. That only a
fragment of the main
explosion reached the ground and made a relatively small crater, without
creating a larger main crater, seems contradictory to Morrison.
Harris
agreed that physics could work against Longo's explanation, but did note that
similar events — with impact craters — have been documented all over the world.
"In 1947,
the Russian Sikhote-Alin meteorite created 100 small
craters. Some were 20 meters (66 feet) across," Harris said. A site in Poland also exists, he explained, where a large meteor exploded and created a series of
small lakes. "If the fragment was traveling slowly enough, there's actually a
good chance (Longo's team) will unearth some meteorite material," Harris said.
Longo's
team plans to return to Lake Cheko next summer, close to the 100th
anniversary of the Tunguska Event. "This is important work because we can make
better conclusions about how cosmic bodies impact the Earth, and what they're
made of," Longo said. "And it could help us find ways to protect our planet
from future impacts of this kind."
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