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2009-03-08T07:20:00.000-07:00

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Tunguska

2008-06-26T10:03:00.000-07:00

The
Tunguska
Mystery

By Luca Gasperini,
Enrico Bonatti
and Giuseppe Longo

Scientific American June 2008

June 30, 1908, 7:14 a.m., central Siberia—Semen Semenov,
a local farmer, saw “the sky split in two. Fire appeared high
and wide over the forest. . . . From . . . where the fire was,
came strong heat. . . . Then the sky shut closed, and a strong
thump sounded, and I was thrown a few yards.... After that such
noise came, as if . . . cannons were firing, the earth shook ...”
Such is the harrowing testimony of one of the closest eyewitnesses
to what scientists call the Tunguska event, the largest
impact of a cosmic body to occur on the earth during modern
human history. Semenov experienced a raging conflagration some
65 kilometers (40 miles) from ground zero, but the effects of the
blast rippled out far into northern Europe and Central Asia as
well. Some people saw massive, silvery clouds and brilliant, colored
sunsets on the horizon, whereas others witnessed luminescent
skies at night—Londoners, for instance, could plainly read
newsprint at midnight without artificial lights. Geophysical
observatories placed the source of the anomalous seismic and
pressure waves they had recorded in a remote section of Siberia.
The epicenter lay close to the river Podkamennaya Tunguska, an
uninhabited area of swampy taiga forest that stays frozen for eight
or nine months of the year.

Ever since the Tunguska event, scientists and
lay enthusiasts alike have wondered what caused
it. Although most observers generally accept that
some kind of cosmic body, either an asteroid or
a comet, exploded in the sky above Siberia, no
one has yet found fragments of the object or any
impact craters in the affected region. The mystery
remains unsolved, but our research team,
only the latest of a steady stream of investigators
who have scoured the area, may be closing in on
a discovery that will change our understanding
of what happened that fateful morning.
The study of the Tunguska event is important
because past collisions with extraterrestrial bodies
have had major effects on the evolution of the
earth. Some 4.4 billion years ago, for example, a
Mars-size planetoid seems to have struck our
young planet, throwing out enough debris to create
our moon. And a large impact may have
caused the extinction of the dinosaurs 65 million
years ago. Even today cosmic impacts are evident.
In July 1994 several astronomical observatories
recorded the spectacular crash of a comet on
Jupiter. And only last September, Peruvian villagers
watched in awe and fright as a heavenly object
streaked across the sky and landed not too far
away with a loud boom, leaving a gaping pit 4.5
meters deep and 13 meters wide.
Using satellite observations of meteoric
“flares” in the atmosphere (“shooting stars”)
and acoustical data that record cosmic impacts
on the surface of the earth, Peter Brown and his
co-workers at the University of Western Ontario
and Los Alamos National Laboratory estimated
the rate of smaller impacts. The researchers have
also extrapolated their findings to larger but rarer
incidents such as the Tunguska event. The
average frequency of Tunguska-like asteroidal
collisions ranges from one in 200 years to one in
1,000 years. Thus, it is not unlikely that a similar
strike could occur during our lifetimes. Luckily,
the Tunguska impact took place in an unpopulated
corner of the globe. Should something
like it explode above New York City, the entire
metropolitan area would be razed. Understanding
the Tunguska event could help us prepare for
such an eventuality and maybe even take steps
to avoid its occurrence altogether.
The first step in preparing ourselves would be
to decide whether the cosmic object that affected
Siberia was an asteroid or a comet. Although the
consequences are roughly comparable in either
case, an important difference is that objects in
the solar system that circle far away from the sun
on long-period orbits before returning, such as
comets, would hit the earth at much greater
velocities than close-orbiting (short-period) bodies,
such as asteroids. A comet that is significantly
smaller than an asteroid thus could release the
same kinetic energy in such a collision. And
observers have much more difficulty detecting
long-period objects before they enter the inner
solar system. In addition, the probability that
such objects will cross the earth’s orbit is low relative
to the probability that asteroids will. For
these reasons, confirmed comet impacts on the
earth are so far unknown. Therefore, if the Tunguska
event was in fact caused by a comet, it
would be a unique occurrence rather than an
important case study of a known class of phenomena.
On the other hand, if an asteroid did
explode in the Siberian skies that June morning,
why has no one yet found fragments?
First Expedition
Part of the enduring mystery of the Tunguska
event harks back to the stark physical isolation
of central Siberia and the political turmoil that
raged in Russia during the early 20th century, a
time when the czarist empire fell and the Soviet
Union emerged. These two factors delayed scientific
field studies for nearly 20 years. Only in
1927 did an expedition led by Leonid Kulik, a
meteorite specialist from the Russian Academy
of Sciences, reach the Tunguska site. When Kulik
got to the site, he was confronted with some
almost unbelievable scenery. Amazingly,
the blast had flattened millions
of trees in a broad, butterfly-shaped swath
covering more than 2,000 square kilometers
(775 square miles). Furthermore, the tree trunks
had fallen in a radial pattern extending out for
kilometers from a central area where “telegraph
poles,” a lone stand of partially burned tree
stumps, still remained. Kulik interpreted this
ravaged landscape as the aftermath of an impact
of an iron meteorite. He then began to search for
the resulting crater or meteorite fragments.
Kulik led three additional expeditions to the
Tunguska region in the late 1920s and 1930s,
and several others followed, but no one found
clear-cut impact craters or pieces of whatever
had hit the area. The dearth of evidence on-site
gave rise to various explanatory hypotheses. In
1946, for instance, science-fiction writer Alexander
Kazantsev explained the puzzling scene by
positing a scenario in which an alien spacecraft
had exploded in the atmosphere. Within a few
years, the airburst theory gained scientific support
and thereafter limited further speculation.
Disintegration of a cosmic object in the atmosphere,
between five and 10 kilometers above the
surface, would explain most of the features investigators
observed on the ground. Seismic observatory
records, together with the dimensions of
the devastation, allowed researchers to estimate
the energy and altitude of the blast.
The lack of an impact crater also suggested
that the object could not have been a sturdy iron
meteorite but a more fragile object, such as a relatively
rare, stony asteroid or a small comet.
Russian scientists favored the latter hypothesis
because a comet is composed of dust particlesand ice, which would fail to produce
an impact crater. Another
explanation for the tumult in the
Tunguska region claimed that the destruction
resulted from the rapid combustion of methane
gas released from the swampy ground into
the air.
Laboratory Models
In 1975 Ari Ben-Menahem, a seismologist at
the Weizmann Institute of Science in Rehovot,
Israel, analyzed the seismic waves triggered by
the Tunguska event and estimated that the energy
released by the explosion was between 10
and 15 megatons in magnitude, the equivalent
of 1,000 Hiroshima atomic bombs.
Astrophysicists have since created numerical
simulations of the Tunguska event to try to
decide among the competing hypotheses. The
airburst of a stony asteroid is the leading interpretation.
Models by Christopher F. Chyba,
then at the NASA Ames Research Center, and his
colleagues proposed in 1993 that the asteroid
was a few tens of meters in diameter and that it
exploded several kilometers above the ground.
Comparison of the effects of nuclear test airbursts
with the flattened pattern of the Tunguska
forest seems to confirm this suggestion.
More recent simulations by N. A. Artemieva
and V. V. Shuvalov, both at the Institute for
Dynamics of Geospheres in Moscow, have envisioned
an asteroid of similar size vaporizing five
to 10 kilometers above Tunguska. In their model,
the resulting fine debris and a downwardpropagating
gaseous jet then dispersed over
wide areas in the atmosphere. These simulations
do not, however, exclude the possibility thatmeter-size fragments may have survived the
explosion and could have struck the ground not
far from the blast.
Late last year Mark Boslough and his team at
Sandia National Laboratories concluded that the
Tunguska event may have been precipitated by a
much smaller object than earlier estimates had
suggested. Their supercomputer simulation
showed that the mass of the falling cosmic body
turned into an expanding jet of high-temperature
gas traveling at supersonic speeds. The model
also indicated that the impactor was first compressed
by the increasing resistance of the earth’s
atmosphere. As the descending body penetrated
deeper, air resistance probably caused it to
explode in an airburst with a strong flow of heated
gas that was carried downward by its tremendous
momentum. Because the fireball would
have transported additional energy toward the
surface, what scientists had thought to be an
explosion between 10 and 20 megatons was
more likely only three to five megatons, according
to Boslough. All this simulation work only
strengthened (and continues to strengthen) our
desire to conduct fieldwork at the Tunguska site.
Trip to Siberia
Our involvement with the Tunguska event
began in 1991, when one of us (Longo) took
part in the first Italian expedition to the site,
during which he searched for microparticles
from the explosion that might have become
trapped in tree resin. Later, we stumbled on two
obscure papers by Russian scientists, V. A.
Koshelev and K. P. Florensky, that reported
their discovery of a small body of water, Lake
Cheko, roughly eight kilometers from the suspected
epicenter of the phenomenon. In 1960
Koshelev speculated that Lake Cheko might be
an impact crater, but Florensky rejected that
idea. Florensky instead believed the lake was
older than the Tunguska event, based on having
found loose sediments as thick as seven meters
below the bottom of the lake.
Word that a lake sat close to ground zero
piqued our interest in mounting a field trip there
because lake-bottom sediments can store a
detailed record of events that occurred in the
surrounding region, the basis of paleolimnological
studies. Although our team knew little of
Lake Cheko, we thought that we could perhaps
apply paleolimnological techniques and find in
the lake’s sediments clues to unravel the Tunguska
mystery, as if the lake were the black box
from a crashed airliner.
A few years later we found ourselves journeying
to Russia in the cargo hold of an Ilyushin Il
20M propeller plane, a onetime aerial spy from
the cold war era. Having found the necessary
funds and having organized our venture in cooperation
with research groups at Moscow State
University and Tomsk State University in Russia
(with the assistance of former cosmonaut Georgi
M. Grechko), we were finally on our way to the
Tunguska region. After the transport carried
most of our Italian team and its equipment to a
military base near Moscow, we flew overnight to
Krasnojarsk, in central Siberia. We then transferred
our equipment and ourselves, plus several
researchers from Tomsk State, into the belly of a
huge Mi 26 heavy-lift helicopter (formerly used
by the military). For six hours we squatted among
our equipment, deafened by the chopper’s twin
turboshaft engines, until we finally reached our
distant goal in the middle of the endless taiga.
After circling the lake’s dark waters warily,
the helicopter hovered precariously above the
swampy lakeside (which was too soft for a landing)
as we jumped down amid a torrential rainstorm.
With eight blades rotating furiously above
our heads, the resulting hurricane of air and
water seemed set to sweep us away when at last
we managed to unload our heavy cargo. With a
roar, the craft lifted upward, and we were left
drenched and exhausted near the edge of the lake,
suddenly immersed in the deep silence of the
Siberian wilderness. Any small relief we felt when
the rain stopped was immediately forgotten as
clouds of voracious mosquitoes descended on us
like massed squadrons of tiny dive-bombers.
On-Site Studies
We spent the next two days organizing the camp,
assembling our survey boat (a catamaran) and
testing our equipment. Our studies would
require a range of technologies, such as acoustic
echo sounders, a magnetometer, subbottom
acoustic profilers, a ground-penetrating radar,
devices to recover sediment cores, an underwater
television camera and a set of GPS receivers
to enable study teams to track their position
with a resolution of less than a meter.
For two weeks after that, our group surveyed
the lake from the catamaran, tormented the
entire time by hordes of mosquitoes and horseflies.
These efforts focused on exploring the sedimentation
and structure of the lake’s subbottom.
Other team members, in the meantime,
busied themselves with their own tasks. With his
ground-penetrating radar, ��Michele Pipan, a geophysicist
at the University of Trieste, gradually
mapped the subsurface structures (some three to
four meters deep) below the 500-meter shore
perimeter. Eugene Kolesnikov, a geochemist at
Moscow State, and his colleagues excavated
trenches in peat deposits near the lake, a tough
job given the resistance of the hard permafrost
layer below the surface. Kolesnikov’s team
searched the peat layers for chemical markers of
the Tunguska event. At the same time, Romano
Serra of Bologna University and Valery Nesvetailo
of Tomsk State collected core samples from
nearby tree trunks to study possible anomalies
in the tree-ring patterns. Meanwhile, high above
us, the aircraft that brought us to Krasnojarsk
returned and circled the region to take aerial
photographs so that we could compare them
with those Kulik made some 60 years before.
We had assumed that the lake-bottom sediments
might contain markers of the Tunguska
event. After completing just a few runs across
Lake Cheko with our high-resolution acoustic
profiler, it became clear that the sediments blanketing
the lake’s bottom were more than 10
meters thick. Some sediment particles had been
transported to the lake by winds, but most of
them came by way of the inflow of the little Kimchu
River that fed Lake Cheko. We estimated
that sediment deposition in a small body of
water that stays frozen for most of the year
would probably not exceed a few centimeters a
year, so such a thick sediment layer might imply
that the lake existed before 1908.
On the other hand, the more we profiled the
lake bottom, the more perplexed we became. It
appeared that the lake, which is about 50 meters
(165 feet) deep in the middle and has steep slopes,
is shaped like a funnel or an inverted cone, a
structure that is difficult to explain. If the lake
were thousands of years old, it would probably
have a flat bottom, the result of fine sediments
gradually filling it up. We also found it hard to
account for the funnel shape using typical erosion-
deposition processes that occur when a
small river meanders across a relatively flat landscape.
Our entire team discussed these questions
during the evenings as we sat under rain tarps,
dining on delicious Russian kasha seasoned liberally
with the bodies of dead mosquitoes.
Soon our time in Tunguska was nearly over.
The expedition members spent the last day frantically
disassembling the boat, packing the
equipment and dismantling the camp. When the
helicopter arrived at noon the next day, we
rushed to load all our stuff and ourselves into
the hovering chopper amid the storm of humanmade
turbulence and finally began our return.
Titillating Evidence
Back in our laboratories in Italy, the three of us
completed processing our bathymetric data,
which confirmed that the shape of Lake Cheko’s
bottom differs significantly from those of other
Siberian lakes, which typically feature flat bottoms.
Most lakes in the region form when water
fills the depressions left after the ubiquitous permafrost
layer melts. The funnellike shape of
Lake Cheko, in contrast, resembles those of
known impact craters of similar size—for
instance, the so-called Odessa crater, which was
created 25,000 years ago by the impact of a
small asteroid in what is now Odessa, Tex.
The idea that Lake Cheko might fill an impact
crater became more attractive to us. But if the
lake is indeed a crater excavated by a fragment
of the Tunguska cosmic body, it cannot have
been formed earlier than 1908. We sought evidence
that the little lake existed before the event.
Reliable, pre-1908 maps of this uninhabited
region of Siberia are not easy to come by, but we
found a czarist military map from 1883 that fails
to show the lake. Testimony by local Evenk
natives also asserts that a lake was produced by
the 1908 explosion. But if the lake was not
formed before 1908, how can one explain the
thickness of the deposits carpeting its floor? Our
seismic-reflection data revealed two distinct
zones in the lake’s deposits: a thin, roughly
meter-thick upper level of laminated��, fine sediments
typical of quiet deposition overlying a
lower region of nonstratified, chaotic deposits.
A recent study by two Italian paleobotanists,
Carla Alberta Accorsi of the University of Modena
and Luisa Forlani of the University of Bologna,
however, has shown that whereas the upper
sediment layers contain abundant evidence of
aquatic plants, these signs are totally absent in
the lower chaotic deposits, which hold plentiful
quantities of pollen from forest trees. So it looks
as if the lake’s true deposits are only about a
meter thick, a feature that is compatible with a
hypothesis that posits a young age for the lake. A
forest seems to have grown on wet ground there
before the lake formed.
Our survey team also observed the half-buried
remains of tree trunks in the deeper part ofthe lake via underwater video. And high-frequency
acoustic waves reflected back from the
same zone showed a characteristic “hairy” pattern
that could have resulted from the presence
of the remains of trunks and branches. Perhaps
these results are a trace of the forest obliterated
by the impact.
Suspect Lake Shape
To explain the lower chaotic deposits, we can
imagine a cosmic body hitting soggy ground
overlying a layer of permafrost several tens of
meters thick. The impactor’s kinetic energy is
transformed into heat, which melts the permafrost,
releasing methane and water vapor and
expanding the size of the resulting crater by as
much as a quarter. At the same time, the impact
would have plastered preexisting river and
swamp deposits onto the flanks of the impact
crater, where they would later be imaged as the
chaotic deposits in our acoustic-echo profiles.
Most intriguing, a careful analysis of the seismic-
reflection profiles we obtained across the
lake has revealed several meters below the deepest
point at the center a strong acoustic reflector,
probably the echo of a dense, meter-size rocky
object. This result is supported by the finding of
a small magnetic anomaly above the same spot
during our magnetometer survey. Are these indications
of a fragment of the Tunguska body?
We are anxious to find out. Our team is now
preparing to return later this year to attempt to
drill the center of the lake to reach the dense seismic
reflector. The year 2008 is the centennial of
the Tunguska event. We hope it will also be the
year the Tunguska mystery is solved. N
© 2008 SCIENTIFIC AMERICAN, INC.
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