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Comment Article Evidence That Lake Cheko Is Not an Impact Crater

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Comment Article Evidence That Lake Cheko Is Not an Impact Crater doi: 10.1111/j.1365-3121.2008.00791.x Comment article Evidence that Lake Cheko is not an impact crater G. S. Collins,1 N. Artemieva,2 K. Wu¨ nnemann,3 P. A. Bland,1 W. U. Reimold,3 and C. Koeberl4 1Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, SW7 2AZ London, UK; 2Institute for the Dynamics of Geospheres, Russian Academy of Sciences, Moscow, Russia; 3Museum for Natural History, Humboldt University, Invalidenstrasse 43, 10115 Berlin, Germany; 4Center of Earth Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria ABSTRACT In a provocative paper Gasperini et al. (2007) suggest that Lake is required for an asteroid fragment to traverse the EarthÕs Cheko, a 300-m-wide lake situated a few kilometres down- atmosphere and reach the surface intact and with sufficient range from the assumed epicentre of the 1908 Tunguska event, velocity to excavate a crater the size of Lake Cheko. Inferred is an impact crater. In this response, we present several lines of tensile strengths of large stony meteorites during atmospheric observational evidence that contradicts the impact hypothesis disruption are 10–100 times lower. We therefore conclude that for the lakeÕs origin: un-crater-like aspects of the lake mor- Lake Cheko is highly unlikely to be an impact crater. phology, the lack of impactor material in and around the lake, and the presence of apparently unaffected mature trees close Terra Nova, 20, 165–168, 2008 to the lake. We also show that a tensile strength of 10–40 MPa over 170 confirmed impact structures The first piece of evidence does give Introduction on the Earth (Earth Impact Database, pause for thought, but could easily be An impact origin for Lake Cheko and 2007). Every new, ÔconfirmedÕ impact coincidence. The second argument is connection with the Tunguska event crater provides important information key: if the lake pre-dates 1908, it was was dismissed by a Russian expedition to further our understanding of obviously not formed during the in the 1960s when a tentative age of impacts and the hazard they pose. Tunguska event. At this stage, there 5–10 ka for the lake was made based However, every false impact crater is no convincing evidence that the lake on the thickness (7 m) of mud depos- that is proposed clouds our under- is only 100 years old; anecdotal evi- its (Florenskij, 1963). On the basis of standing and confuses the public (Rei- dence cannot be relied on, and recent new geophysical data and shallow lake mold, 2007), and for this reason the collapse features do not imply that the sediment cores, Gasperini et al. (2007) burden of proof in identifying a new lake formed recently. The third piece argue against such an old age for the impact structure must lie with the of evidence is the weakest; geophysical lake. They hypothesize that a 10-m- proponents. Disappointingly, in the anomalies can be interpreted in many diameter fragment of the Tunguska case of Lake Cheko, very little evi- ways and never provide conclusive asteroid or comet survived the main dence has been supplied in support of evidence of impact. For the bright atmospheric disturbance, continued on the impact hypothesis for its origin, reflector to be caused by the impacting its course to collide with the ground at and none of it is compelling. Gaspe- body implies an unrealistically large a velocity of many kilometres per rini et al. (2007) provide four argu- and robust impactor, to survive second, and formed Lake Cheko. ments in support of an impact origin impact intact and be resolvable in Impact cratering is an important for the lake: the seismic data. It is far more likely geological process that has cata- that the bright reflector is sedimentary 1 The location of the lake is consis- strophically affected the global envi- in origin. Finally, the morphology of tent with the continuation of the ronment. Much of our current the lake is, in many ways, very differ- assumed trajectory of the Tunguska knowledge of the impact process and ent from small impact craters on impactor beyond the epicentre of hazard is derived from the study of Earth, as we discuss below, and there- the explosion. fore does not provide compelling evi- 2 The age of the lake is unknown. dence for an impact origin. Correspondence: Gareth S. Collins, Depart- 3 A bright seismic reflection is appar- ment of Earth Science and Engineering, ent in the seismic data beneath the Imperial College London, South Kensing- lake. The authors claim that this The Tunguska Event ton Campus, SW7 2AZ London, UK. might be evidence for impactor Years of theoretical, analogue and Tel.: +44 (20) 75941518; fax: +44 (20) material or impact-compaction of numerical modelling work have been 75947444; e-mail: [email protected] the sediments. devoted to explaining the major 4 The lake has a funnel-like morphol- consequences of the Tunguska event A response to: A possible impact crater for ogy. The authors claim this is on 30th June 1908. The most recent the 1908 Tunguska Event L. Gasperini, F. unusual for the area and similar to numerical model, which agrees with Alvisi, G. Biasini, E. Bonatti, G. Longo, M. other small impact craters on Earth. Pipan, M. Ravaioli and R. Serra. the consensus of earlier models Ó 2008 Blackwell Publishing Ltd 165 Evidence that Lake Cheko is not an impact crater • G. S. Collins et al. Terra Nova, Vol 20, No. 2, 165–168 ............................................................................................................................................................. (Korobeinikov et al., 1976; Chyba but this would occur close to, and up the form of iron meteorite fragments et al., 1993; Boslough and Crawford, range of, the epicentre. or Fe-Ni metal grains and spherules 1997), suggests that the Tunguska For a high-velocity impact crater to has been found (Table 1). No trace of event was caused by a cosmic body form as part of the atmospheric dis- impactor material has yet been found 50–80 m in diameter entering the turbance over Tunguska in 1908, in or around Lake Cheko, and if it EarthÕs atmosphere at 20 km s)1 and therefore, is contrary to our current exists it is extremely unlikely to be at an inclination of 30–45° to the understanding of the event. Neverthe- iron. If Lake Cheko is a 300-m- horizontal (Artemieva and Shuvalov, less, Gasperini et al. (2007) estimate diameter impact crater formed by a 2007). At an altitude of 20 km, the that a 10 m diameter1 Asteroid frag- stony meteorite, it would be anoma- impactor starts to deform, disrupt and ment impacting at a velocity of 1– lous in terms of its impactor compo- evaporate strongly; the resulting jet of 10 km s)1 is required to form an sition. vaporized impactor material is totally impact crater the same size as Lake Another important observation is decelerated at an altitude of 8–10 km Cheko. For this putative impactor to that all young (<10 ka), 0.1- to 0.3- and releases all its energy into the survive passage through the atmo- km-diameter meteorite craters are not atmosphere (5–15 Mt, high explosive sphere from an altitude of 8km isolated craters, but are part of crater equivalent, consistent with estimates (the assumed height of the air blast Ôstrewn fieldsÕ, formed by the near- from seismic records Ben-Menahem, centre) to the ground, the fragment simultaneous collision of a number of 1975). The mixture of hot air and must have a tensile strength of at least dispersed fragments of the same ori- vaporized impactor material is buoy- 10 MPa if it strikes the ground at ginal meteoroid (Table 1). Given the ant and accelerates back along the 5kms)1, and of at least 40 MPa if it obvious disruption that occurred to wake, while an atmospheric shock strikes the ground at 10 km s)1. the bulk of the Tunguska impactor, it wave reaches the surface. The interac- Although these values are within the is very hard to explain how Lake tion of the shock wave with the range of laboratory measurements of Cheko could have formed by impact surface causes the observed famous tensile strengths of cm-sized meteorite in isolation. If Lake Cheko is an forest devastation and fallen tree samples (Grady and Lipkin, 1980; impact crater, and depending on the pattern (Florenskij, 1963); the model Medvedev et al., 1985; Petrovic, composition of the impactor and exact results are consistent with observa- 2001), the dynamic strength of larger impact velocity, a number of solid tions of total area and ÔbutterflyÕ plan- objects (1–20 cm), observed disrupt- fragments of the impactor should be form of the damage area, and the ing as they enter the EarthÕs atmo- preserved in and around the lake. telegraph poles and trees that sphere, are substantially lower, rarely remained vertical near the epicentre. exceeding 1 MPa (Ceplecha et al., Other observational evidence All the extra terrestrial impactor 1993). The low strength of stony material (in the form of tiny droplets) objects is further indirectly confirmed In addition to hard evidence of impact is carried ÔuprangeÕ away from the by an absence of large stony meteor- that is lacking for Lake Cheko, there epicentre, reaches high altitudes and ites on the Earth. Hence, the impact are several observable differences may then disperse worldwide. It is hypothesis for Lake Cheko requires between the lake and small meteorite most likely that this dispersal of hot that the impactor must have been an impact craters on the Earth. Gasperini condensed impactor material caused exceptionally strong and large frag- et al.
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