Lake Cheko and the 1908 Tunguska Event

The so-called Tunguska Event (TE), of June 30 1908, is one of those phenomena that have generated a great and durable interest into generations of earth and planetary scientists and a worldwide audience. This is due to several causes, including the remoteness of the site where it occurred and the global scale of its effects. Thousands of kilometers from the epicentre, over Russia and the Northern Europe, the TE was perceived as a series of unusual phenomena, such as seismic and pressure waves and a bright luminescence in the night skies. Only several years later, in 1927, Leonid Kulik reached the explosion site, a remote region of Central Siberia close to the river Podkamennaya Tunguska, which gave later the name to the event. Kulik identified the epicentre of the explosion in a heavily forested area from the radial distribution of flattened trees. He also found many trees that were left standing directly below the explosion, although deprived of their branches and partially burned. Kulik reached the conclusion that he had discovered the remains of a large impact crater hidden by a swamp and a number of secondary bowl-shaped holes of different sizes covered by peat bogs. This pattern could have been caused by an asteroid that fell in a swarm of separate fragments. Other authors questioned this interpretation, suggesting that the observed circular features were not necessarily related to extraterrestrial impacts, but probably to seasonal thawing and freezing of the ground. Two observations favoured this second interpretation: circular features of similar size are common at these latitude; these depressions were found in low-lying areas, but not on the slopes of the hills, or on any of the reliefs occupying a considerable part of the epicentre region. Moreover, all attempts at finding macro-remnants of the cosmic body by digging 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 the tree patterns in the devastated taiga and to the search for micro particles of the cosmic body, under the assumption that it exploded in the atmosphere. After more than a century, the TE is far from being completely understood, although it has been the object of several studies. The most probable hypothesis is that it was caused by the impact with the Earth of a cosmic body, a comet or an asteroid, which exploded about 5-10 km above the ground, releasing in the atmosphere 10-15 Mtons of energy. However, fragments of the impacting Tunguska Cosmic Body (TCB) have never been found, and its nature is still a matter of debate. Geophysical and geological data collected from a small lake about 8 km NNW of the inferred TE epicentre suggest that it is probably a crater left by the impact of a large TCB fragment. Evidence supporting this interpretation includes its funnel-like bottom morphology, revealed by high-resolution bathymetry, and the structure and composition of sedimentary deposits below the lake floor, studied through acoustic imagery and direct sampling. Seismic reflection profiles collected from the lake show a density/P-wave velocity anomaly located about 10 m below its bottom, probably the effect of a buried object, which could have created the lake's depression, or a density anomaly caused by overpressure due to the impact. A magnetic anomaly also marks the presence of such feature. If Lake Cheko, as much evidence seems to suggest, was formed as a consequence of an extraterrestrial impact, it opens the problem of recognizing impact-related features on a composite target such as the Earth surface, very different from the Moon and the other terrestrial planets of the Solar System, where the morphology of "typical" impact features has been mostly studied.