A mystery solved: Space shuttle shows 1908 Tunguska explosion was caused by comet (Forwarded)

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June 24, 2009

A mystery solved: Space shuttle shows 1908 Tunguska explosion was caused by
comet
By Anne Ju

The mysterious 1908 Tunguska explosion that leveled 830 square miles of
Siberian forest was almost certainly caused by a comet entering Earth's
atmosphere, says new Cornell research. The conclusion is supported by an
unlikely source: the exhaust plume from the NASA space shuttle launched a
century later.

The research, accepted for publication (June 24) by the journal Geophysical
Research Letters, published by the American Geophysical Union, connects the
two events by what followed each about a day later: brilliant, night-visible
clouds, or noctilucent clouds, that are made up of ice particles and only
form at very high altitudes and in extremely cold temperatures.

"It's almost like putting together a 100-year-old murder mystery," said
Michael Kelley, the James A. Friend Family Distinguished Professor of
Engineering at Cornell, who led the research team. "The evidence is pretty
strong that the Earth was hit by a comet in 1908." Previous speculation had
ranged from comets to meteors to black holes.

The researchers contend that the massive amount of water vapor spewed into
the atmosphere by the comet's icy nucleus was caught up in swirling eddies
with tremendous energy by a process called two-dimensional turbulence, which
explains why the noctilucent clouds formed a day later many thousands of
miles away.

Noctilucent clouds are the Earth's highest clouds, forming naturally in the
mesosphere at about 55 miles over the polar regions during the summer months
when the mesosphere is around minus 180 degrees Fahrenheit (minus 117
degrees Celsius).

The space shuttle exhaust plume, the researchers say, resembled the water
vapor from the comet. A single space shuttle flight injects 300 metric tons
of water vapor into the Earth's thermosphere, and the water particles have
been found to travel to the Arctic and Antarctic regions, where they form
the clouds after settling into the mesosphere. The thermosphere is the layer
of the atmosphere above the mesosphere.

Kelley and collaborators saw the noctilucent cloud phenomenon days after the
space shuttle Endeavour (STS-118) launched on Aug. 8, 2007. Similar cloud
formations had been observed following launches in 1997 and 2003.

Following the 1908 explosion, known as the Tunguska Event, the night skies
shone brightly for several days across Europe, particularly Great Britain --
more than 3,000 miles away. Kelley said he became intrigued by the
historical eyewitness accounts of the aftermath, and concluded that the
bright skies must have been the result of noctilucent clouds. The comet
would have started to break up at about the same altitude as the release of
the exhaust plume from the space shuttle following launch. In both cases,
water vapor was injected into the atmosphere.

The scientists have attempted to answer how this water vapor traveled so far
without scattering and diffusing, as conventional physics would predict.

"There is a mean transport of this material for tens of thousands of
kilometers in a very short time, and there is no model that predicts that,"
Kelley said. "It's totally new and unexpected physics."

This "new" physics, the researchers contend, is tied up in counter-rotating
eddies with extreme energy. Once the water vapor got caught up in these
eddies, the water traveled very quickly -- close to 300 feet per second.

Scientists have long tried to study the wind structure in these upper
regions of the atmosphere, which is difficult to do by such traditional
means as sounding rockets, balloon launches and satellites, explained
Charles Seyler, Cornell professor of electrical engineering and paper
co-author.

"Our observations show that current understanding of the mesosphere-lower
thermosphere region is quite poor," Seyler said.

The paper is also co-authored by Clemson University physicist Miguel Larsen,
Ph.D. '79, a former student of Kelley. The work performed at Cornell was
funded by the Atmospheric Science Section of the National Science
Foundation.

On July 1, Kelley will give a lecture, "Two-dimensional Turbulence, Space
Shuttle Plume Transport in the Thermosphere, and a Possible Relation to the
Great Siberian Impact Event," at a plenary session of the annual meeting of
Coupling, Energetics and Dynamics of Atmospheric Regions in Santa Fe, N.M.

The paper is available at
http://www.agu.org/journals/gl/papersinpress.shtml

IMAGE CAPTION:
[http://www.news.cornell.edu/stories/June09/cloud.jpg (10KB)]
Noctilucent clouds observed from Donnelley Dome near Fairbanks, Alaska,
resulting from a post-space shuttle plume in August 2007 (M.J. Taylor and
C.D. Burton/Utah State University).