[meteorite-list] A Possible Impact Crater for the 1908 Tunguska Event

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 22 Jun 2007 09:08:20 -0700 (PDT)
Message-ID: <200706221608.JAA08324_at_zagami.jpl.nasa.gov>

http://www.blackwell-synergy.com/doi/full/10.1111/j.1365-3121.2007.00742.x

Terra Nova

To cite this article: L. Gasperini, F. Alvisi, G. Biasini, E. Bonatti,
G. Longo, M. Pipan, M. Ravaioli, R. Serra

A possible impact crater for the 1908 Tunguska Event

    * L. Gasperini, ISMAR-CNR, Sezione di Geologia Marina, Bologna, Italy,
    * F. Alvisi, ISMAR-CNR, Sezione di Geologia Marina, Bologna, Italy,
    * G. Biasini, Communication Technology, Cesena, Italy,
    * E. Bonatti, ISMAR-CNR, Sezione di Geologia Marina, Bologna, Italy,
    * G. Longo, Dipartimento di Fisica, Universita' di Bologna,
    * M. Pipan, Dipartimento di Scienze della Terra, Universita' di Trieste,
    * M. Ravaioli, ISMAR-CNR, Sezione di Geologia Marina, Bologna, Italy and
    * R. Serra, Dipartimento di Fisica, Universita' di Bologna

Luca Gasperini, Geologia Marina, Istituto di Scienze Marine, CNR, Via
Gobetti 101, Bologna 40129, Italy.
Tel.: +39 051 639 8901; fax: +39 051 639 8901; e-mail:
luca.gasperini at ismar.cnr.it . Re-use
of this article is permitted in accordance with the Creative Commons
Deed, Attribution 2.5, which does not permit commercial exploitation.

Terra Nova, 00, 1-7, 2007

Abstract

The so-called "Tunguska Event" refers to a major explosion that occurred
on 30 June 1908 in the Tunguska region of Siberia, causing the
destruction of over 2000 km2 of taiga, globally detected pressure and
seismic waves, and bright luminescence in the night skies of Europe and
Central Asia, combined with other unusual phenomena. The "Tunguska
Event" may be related to the impact with the Earth of a cosmic body that
exploded about 5-10 km above ground, releasing in the atmosphere
10-15 Mton of energy. Fragments of the impacting body have never been
found, and its nature (comet or asteroid) is still a matter of debate.
We report results from the investigation of Lake Cheko, located ~8 km
NNW of the inferred explosion epicenter. Its funnel-like bottom
morphology and the structure of its sedimentary deposits, revealed by
acoustic imagery and direct sampling, all suggest that the lake fills an
impact crater. Lake Cheko may have formed due to a secondary impact onto
alluvial swampy ground; the size and shape of the crater may have been
affected by the nature of the ground and by impact-related melting and
degassing of a permafrost layer.

Introduction

Unusual phenomena were detected on 30 June 1908 over Eurasia. They
included seismic and pressure waves recorded at several observatories;
bright luminescence in the night skies; anomalous optical phenomena in
the atmosphere, such as massive glowing silvery clouds and brilliant
colorful sunsets (Busch, 1908; Zotkin, 1961; Vasilyev et al., 1965).
These phenomena were later interpreted as
being caused by the explosion of a cosmic body in a remote region of the
Central Siberia, close to the river Podkamennaya Tunguska, where
eyewitnesses observed a huge fireball crossing the sky from the SE. This
is the so-called "Tunguska Event", an explosion that is thought to have
released from 10 to 15 Mton of energy in the atmosphere (Ben-Menahem,
1975) and is a major event of this kind in historical times.

Several expeditions explored the Tunguska site, starting with those led
by Leonid Kulik in the late 1920s and 1930s. Kulik identified the
epicenter of the explosion in a heavily forested area from the radial
distribution of flattened trees, and concluded that he had discovered
the remains of a large impact crater now hidden by a swamp (Fig. 1).
He also found a number of secondary
bowl-shaped holes of different sizes covered by peat bogs possibly
caused by a fragmented body that fell in a swarm (Kulik, 1933, 1940).
Other authors questioned this
interpretation suggesting that the circular features observed in the
area of the epicenter were not necessarily related to extraterrestrial
impacts, but probably to seasonal thawing and freezing of the ground,
characterized by a permafrost layer as thick as ~30 m (Krinov, 1949).
All attempts at finding macro-remnants of
the cosmic body in these circular depressions were unsuccessful;
therefore, the hypothesis of an impact with the ground was abandoned.
Subsequent expeditions have been devoted mainly to the study of tree
patterns in the devastated taiga and to the search for micro-particles
of the cosmic body, under the assumption that it exploded 5-10 km above
the ground (Florenskij, 1963).

Lake Cheko, a small lake located close to the inferred Tunguska Event
epicenter (Fig. 1 ), was the focus of a
geological/geophysical expedition that took place in July 1999 (Longo
et al., 2001). The objective of the study was
to search the lake deposits for possible geochemical and
sedimentological markers of the event. However, as the work progressed,
a second objective arose, namely, to find evidence pro or contra the
hypothesis that the lake might fill an impact crater.

Investigation of Lake Cheko

Previous information on Lake Cheko was limited to few soundings and
sediment samples collected in 1960 (Koshelev, 1963). Ho
However, as the region is remote and
uninhabited, there is no reliable evidence even on whether or not the
lake existed before 1908. In fact, the presence of the lake was not
reported in maps drafted before 1928 and is not mentioned by eyewitness
testimonies (Vasilyev et al., 1981). Aerial
images and digital terrane models collected during our 1999 expedition
show that the lake is located within an alluvial plain covered by
sedimentary deposits of the river Kimchu, that flows into the lake on
its SW side and outflows ~200 m away on the same side (Fig. 2).
The eastern shore of the lake is partially
bounded by a hill made of igneous rocks, part of the pre-Mesozoic
regional basement (Sapronov, 1986). The
river, like other rivers in this region, displays wide meanders due to
the low topographic gradient.

We studied the lake bottom morphology using a 200 kHz echo-sounder and a
side-scan sonar system, while the internal structures of the lake
sediments were imaged by mini-seismic reflection profilers, the
low-frequency DataSonics "Bubble-Pulser", and the higher frequency
(high-resolution) "ChirpII" subbottom profiler. Sediment cores up to
1.8 m long were collected using a gravity corer. In addition, a Ground
Penetrating Radar (GPR) was used in the vicinity of the lakeshores to
integrate the seismic grid and to link sub-aerial and sub-lacustrine
stratigraphy. Profiles and samples were positioned through a DGPS
receiver, with an accuracy of +/-l m.

The lake, if we exclude a shallow (<2 m deep) flat area on its SE side,
has a nearly circular shape, slightly elongated in the SE-NW direction
(125?), and a funnel-like morphology, with a ~50 m maximum water-depth
close to its geometrical center (Figs 3 and 4). The slopes
are slightly asymmetrical, the northern being a little steeper than the
southern and do not show important morphological breaks. The main
irregularities are related to sedimentary features and are localized in
two areas, the northern slope where a small mound (probably a slump)
rises from the lake depocenter, and the SW sector, where the inflowing
Kimchu river forms a small lacustrine delta; here, a sharp unconformity
marks the onset of lacustrine over older alluvial/fluvial deposits (U1,
Fig. 5). Processes causing these two types of
features, i.e. sedimentary-wedge progradation and gravity failure, are
likely to occur within short time scales, the former within decades or
centuries, and the latter within seconds; therefore, their occurrence is
compatible with a recent formation of Lake Cheko.

Our seismic-reflection profiles revealed a complex depositional setting
within the lake. We observed an irregular pattern, with geometries
varying from steeply dipping to chaotic, below a thin (0.5-2 m) finely
layered sub-horizontal sequence (Figs 5 and 6). Low-frequency
seismic profiles display a single flat strong reflector (reflector-T,
Fig. 6) close to the lake center that appears
to be produced by a localized discontinuity because it originates from a
wide hyperbola visible in the unmigrated section (Fig. 6b).
Our single-channel system does not allow to
estimate seismic velocities above and below this reflector. However,
after time-migration processing, a clear reflector is visible ~10 m
below the lake bottom, marking the presence of a sharp density/velocity
contrast.

GPR profiles collected in the shallow south-eastern sector confirm a
recent onset of lacustrine condition (Fig. 7),
while side-scan sonar images reveal the presence at the lake bottom of
alternate bands of high and low reflectivity that can be due to annular
fractures, probably diagnostic of gravity slope-failures and collapse
towards the lake center (Fig. 8).

Sediment cores support the geophysical observations in so far as they
show the upper portion of the sedimentary column made of dark, well
laminated, organic-rich lacustrine mud, overlying massive/chaotic
deposits (Fig. 9).

Origin of Lake Cheko

We review some possibilities for the origin of Lake Cheko:

1. In a hypothetical pre-lake scenario, the river Kimchu would have
excavated a major meander and the inverted conical depression as it
approached the basement relief, continuing then its course on a SE-NW
direction, i.e. downstream the present outflowing river (Fig. 2).
We find it highly unlikely that the river
"normal" erosion/redeposition processes could have created the ~50 m
deep, inverted/conical depression presently filled by the lake. We find
it equally difficult to explain the Cheko depression by limestone karsic
chemical erosion, since limestones are absent, or by basement
faulting/fissuring, because the lake is within a tectonically stable
cratonic region.

2. Another possibility is that the lake filled a volcanic crater
intercepted by a river meander during its migration. The region affected
by the Tunguska Event is centred on the roots of the lower Triassic
Kulikovsky palaeo-volcanic complex, which extends over a 400 km2 wide
area displaying numerous, various sized craters (Sapronov, 1986).
The Cheko depression, however, stands above
the alluvial plain deposits of the Kimchu river, as shown by maps and
seismic profiles (Figs 4 and 5). A topographic "hole" such as
the Lake Cheko would be completely filled by fluvial sediments in a
fraction of the age of the volcanic craters observed in the region.
Moreover, the rocks outcropping in the vicinity of the lake are not
eruptive, but mostly dolerites and microgabbros.

3. A large number of lakes have been generated in the subarctic region
of Siberia by thermokarst, i.e. the process by which permafrost may
become unstable and melt, resulting in water-filled depressions of the
ground. Thermokarst lakes are characterized by steep slopes and nearly
flat floors, quasi-circular shapes, with diameter up to several hundred
metres (Czudek and Demek, 1970). The inverted
conical morphology of Cheko, with -50 m water-depth near the centre,
makes a thermokarst origin unlikely. Topographic profiles of Lake Cheko
and of a Siberian thermokarst lake (Lake Nikolaji in the Lena Delta
region) compared with a cross-section of a terrestrial impact crater
(the Odessa Meteor Crater, in Texas) show that the two lakes are
completely different, while the morphology of Cheko resembles that of
the Odessa Meteor Crater (Fig. 10).

Is Lake Cheko an impact crater?

Attempts have been made to determine the trajectory of the cosmic body
responsible for the Tunguska Event, based on eyewitness accounts,
modelling of the ballistic wave and patterns in the devastated forest.
Earlier estimates, although differing from each other, are averaged
around 110? (Sekanina, 1998), while more
recent reconstructions based on eyewitness accounts (Andreev, 1990)
and patterns in the devastated forest (Fast
et al., 1976); Fast and Golenberg, 1983)
led to estimates of 120??20? and 99??10?
respectively. A new analysis based on tree patterns suggests two
azimuths: 110?, for a single explosion scenario, and 135? under the
assumption of multiple centers (Longo et al., 2005).
These azimuths are close to the 125?
orientation of the elliptical Cheko depression (Fig. 4).
Moreover, the lake is located along the
prolongation from the epicenter of the most probable track of the cosmic
body (Fig. 1). Given the above, and given the
difficulty to explain the lake by thermokarst or by "normal" river
sedimentation/erosion processes, we now discuss a scenario whereby Lake
Cheko formed as a result of the impact of a cosmic body in a swampy
taiga-covered area, close to a major meander of the Kimchu River.

Several lines of evidence indicate that the Tunguska Event was caused by
the explosion of a main body 5-10 km above ground (Florenskij, 1963);
one or more fragments of the body may have
survived the main explosion and impacted the ground NW of the epicentre
(Artemieva and Shuvalov, 2007). Many
trustworthy eyewitnesses heard multiple explosions (Kulik, 1927);
moreover, fallen trees pattern based on the
1938 aero-photo survey suggested the presence of two to four secondary
centres of wave propagation (Kulik, 1940),
implying possible multiple centres of explosion (Goldine, 1998).

Small, bowl-shaped impact craters on Earth all have similar geometries,
i.e. a deep cavity with a typical depth-to-diameter ratio (~1:3) and an
overturned flap of ejected material around the rim (Melosh, 1989).
Lake Cheko fits these proportions, although
with a relatively low depth-to-diameter ratio (~0.16) that suggests a
"wet" target (Kenkmann et al., 2007) but
lacks an overturned flap of ejecta. Moreover, it is slightly elliptical
in shape. Elliptical craters result either from low-velocity
(0.5-10 km s-1), moderately oblique (30-60?) impacts, or from extremely
oblique (<10?) higher-velocity impacts. In order to form a ~300-m
diameter crater within the first scenario, scaling laws require an
impactor with a 10-50 m diameter (Melosh, 1989).
The upper limit is not realistic, being
very close to pre-atmospheric entry size estimated for the Tunguska
bolide. The low-velocity suggests that the bulk of the impactor may have
survived the collision and, if so, should be buried below the lake.
Concerning this point, reflector-T observed in profile BP-18 (Fig. 7)
is compatible with the presence of a buried
object or a compacted sedimentary layer below the centre of the lake.
          
The effect of permafrost melting and H2O release at impact

The morphology of the lake floor and subbottom images of the sedimentary
sequence are compatible with the hypothesis of a 10-m diameter stony
object impacting the ground with relatively low velocity (1-10 km s-1),
and impact angle (<=45? over the horizontal). A probable scenario implies
that a single fragment survived the airburst, continued along its
trajectory and impacted down range of the air blast epicentre.

Estimates of the size of the impacting body derived from the size of the
crater are affected by the nature of the ground where the impact took
place. In the Lake Cheko case, it consisted of a H2O-logged, swampy and
forested taiga underlained by a layer of permafrost ranging up to 25 m
in thickness. In addition to its mechanical effect, the impact must have
caused a strong thermal effect that may have melted the permafrost layer
in the vicinity of the impact, with a volume reduction of the ground
material mainly due to evaporation and/or drainage of interstitial H2O,
and degassing of CH4. Subarctic Siberian permafrost stores large
quantities not only of H2O-ice, but also of CH4, partly derived from the
decay of ancient Pleistocene organic matter; Siberian lakes are a major
source of CH4 to the atmosphere (Zimov et al., 1997, 2006);
Walter et al., 2006).
Assuming a 10-m diameter stony object
(density 3000 kg m-3) impacting the ground with a speed of 10 km s-1, we
obtain 0.8 ? 1014 J of kinetic energy released by the impact. It has
been estimated that, in an average impact case, ~1/2 of the kinetic
energy is transferred to the ground (Melosh, 1989).
This amount depends on several parameters,
including the strength and the nature of the target. Due to the soft
nature of the swampy taiga we expect an efficient energy transfer to the
ground. However, assuming conservatively that 0.4 ? 1014 J were
transferred to the ground, ~25% of the total crater volume may have
melted, thus enhancing significantly its final dimensions.

This scenario, i.e. the formation of a crater due to the "soft" impact
of a small body, subsequently enlarged by the expulsion of H2O and gas
from the ground, would explain the unusual morphological/stratigraphical
features observed in the lake. It would also explain the limited
air-blast effects in the lake surroundings, and the absence of a rim
that, if formed during the impact would have been rapidly obliterated by
collapse and gravity-failures during the subsequent degassing phase.
Moreover, it would explain the presence in the bottom of the lake of a
chaotic/massive sediment unit below a well-layered "normal" fine grained
lacustrine sedimentary sequence. Our cores (max 1.80 m) did not reach
the impact level and the pre-impact sediments, and do not allow us to
confirm or reject our hypothesis. Obtaining longer cores of the lake
sediments will be crucial to verify our reconstruction.

Conclusions

Cheko, a small lake located 8 km from the alleged epicentre of the 1908
Tunguska Event, has an unusual funnel-like bottom morphology, with ~50 m
maximum water-depth near the center and a 0.16 depth-to-diameter ratio.
This morphology is different from that of subarctic Siberian thermokarst
lakes, and is also hard to be accounted for other "normal" Earth-surface
tectonic or erosion/deposition processes, but is compatible with the
impact of a cosmic body. Based on diameter, depth and morphology of the
lake crater, and assuming that the impacting object was an asteroid, a
mass of 1.5 ? 106 kg (~10 m diameter) was estimated for the projectile.
However, this estimate is probably too large, because the crater was
enlarged by permafrost melting and release of H2O, CH4 and other
volatiles induced by the impact into a soggy ground. The projectile that
formed Lake Cheko might have been a fragment of the main body that
exploded in the atmosphere 5-10 km above ground. A prominent reflector
observed in seismic reflection profiles ~10 m below the bottom at the
center of the lake indicates a sharp density/velocity contrast,
compatible with either the presence of a fragment of the body, or of
material compacted by the impact. Drilling could solve this dilemma.

Acknowledgements

We are grateful to the Tunguska99 Team, and in particular to G. Andreev,
M. Di Martino, M.Sacchi, L. Vigliotti and P. Zucchini for their help
during the different stages of the present research. We gratefully
acknowledge Dr Dallas Abbot and two anonymous referees who provided
useful comments and suggestions for improving the paper. Most figures
were generated using the GMT software (Wessel and Smith, 1991).
Received on Fri 22 Jun 2007 12:08:20 PM PDT