THE TUNGUSKA METEORITE: A DEAD-LOCK OR THE START OF A NEW STAGE OF INQUIRY? — Part I
The term "Tunguska meteorite fall" refers to the cosmic phenomenon that was observed on June 30, 1908, about 7 a.m. of the local time in Central Siberia, over Krasnoyarsk Territory, Irkutsk Region and Yakutiya [I ]. The most remarkable feature of the event was an explosion of a space object of unknown origin which was moving generally SE to NW and was seen in many settlements of the region.
The flight of the object was attended by sound, seismic and electrophonic effects which covered a vast territory [1-4], and it was equal to the whole set of manifestations of a bolide of -22m --17m of the stellar magnitude. Its brightness was comparable to that of the Sun, but there was no smoky trail characteristic of large iron meteorites. Still, many witnesses noticed a trail of iridescent bands looking like a rainbow [4].
At the moment when the body was flying at the approximate altitude of 5.5 to 8 km over the area with the coordinates 60° 53' N, 101° 54' E (70 km to NW from the little trading station of Vanavara, Krasnoyarsk Territory, not far from the Podkamennaya Tunguska river), there occurred an explosion, or, more precisely, explosion-like energy release.* The TNT equivalent of the effect is estimated as 10-40 megatons, the energy being 4.2? 1023 to 1.7? 1024 ergs [5-8]. There is some evidence suggesting that following the explosion-like energy release at least a part of the Tunguska Space Body (TSB) continued to move in the "pre-explosion" direction upwards [9; 10].
The TSB "explosion" gave rise to a seismic wave recorded in Irkutsk, Tashkent, Tbilisi and Jena [1], as well as pressure disturbances which traveled around the globe [3; 11; 12]. In addition, 5.9+0.9 min. (or, according to another estimate, 6.6 + 0.2 min) after the ''explosion", local magnetic storm began which persisted for more than four hours and was similar to geomagnetic disturbances following nuclear explosions in the atmosphere [13-16].
The shock wave of the Tunguska explosion leveled 2150 + 25 km2 of taiga [9; 17], and the flash burnt vegetation over an area of about 200 km [18; 19]. The Tunguska explosion resulted in a major forest fire covering an area comparable with that of fallen forest [20; 21].
It is noteworthy that the explosion on the Pod-kamennaya (Stony) Tunguska was just the most striking event in the set of natural anomalies which occurred in the summer of 1908 and were probably interrelated. It is now known [22] that beginning on June 23, 1908, atmospheric optical anomalies were observed in many places of Western Europe, the European part of Russia and Western Siberia. They gradually increased in intensity till the 29th of June and then jumped to their peak in the early morning of the 1st of July. These anomalies which are described in detail in Refs. [22 ] and [23] included unprecedentedly active formation of mesospheric (silvery) clouds, bright "volcanic" twilights ("variegated afterglows"), disturbances of the normal travel of the Arago and Babinet neutral points, supposed increase in the emission of the night sky, and unprecedentedly intense and long solar halos. Later, after July 1, these effects exponentially reduced; still some after-effects took place up to late July of 1908.
The area involved in these phenomena included a considerable part of the Northern Hemisphere and was bounded by the Yenisey river in the East, by the Tashkent - Stavropol - Sevastopol - Bordeaux line in the South, and by the Atlantic coast in the West. In August of the year the Western Hemisphere saw a decrease in the air's transparency [24 ] which was due, as V.G.Fesenkov believed, to atmospheric circulation of the Tunguska explosion products. It has been shown recently that along with the Tunguska aerosol cloud, there was simultaneous circulation of products of destruction of another large bolide which entered the Earth's atmosphere in May of 1908 [25]. Superimposition of these effects makes their separate interpretation more difficult.
It should be also noted that the summer of 1908 was quite rich in bright bolides in Siberia, as well as elsewhere [22; 26] (if one compares that year with the years 1907 and 1909 [27]).
The Tunguska meteorite explosion area was found in the 20-s by surface explorations of L.A.Kulik, as well as through analysis of geophysical data performed by A.V.Voznesensky [28]. The chronology and findings of subsequent studies of this area are described in Refs. [29-32].
The expeditions before World War II that were headed by L.A.Kulik [1 ; 2; 33], as well as post-war field works (since 1958 till now) found no explosion-or impact-induced astroblemes or large fragments of the TSB [34-37]. Active search for finely-dispersed space material in the area of the catastrophe, over 10,000 km2 , did not result in discovery of a material to be reliably identified with that of the Tunguska meteorite. The meteoritic dust and similar particles which were found on the site [38-41; 44] cannot be now reliably differentiated from fluctuations of the background fall ofexiraterrestrial matter. However, there have been revealed in the area of the catastrophe some biogeochemical elemental and iso-topic anomalies which may be related to the event under discussion [42—47]. Interpretation of these anomalies is still more complicated since the epicenter of the Tunguska explosion almost ideally coincides with the center of an ancient volcano [48], whose lava flows and thermal ejections undoubtedly affected essentially formation of the biogeochemical situation in the region. The post-war expeditions revealed a complex set of ecological consequences of the Tunguska catastrophe, namely:
1) accelerated growth of young (postcatastrophic) trees and those which survived the event [49-51];
2) population-genetic effects, chiefly at the epicenter and along the TSB trajectory [52; 53].
This is a general outline of the Tunguska phenomenon which, upon thorough study, proves to be principally different from other impact phenomena.
The many hypotheses proposed to explain the Tunguska phenomenon can be subdivided into two groups. One will include those based on the concept of transfer of the kinetic energy of the Tunguska body into the shock wave energy. The other group consists of hypotheses emphasizing release of the internal energy of the body, chemical or nuclear.
Among the hypotheses of the former group, worthy of detailed consideration are primarily those involving concepts of asteroidal (an iron or stone asteroid, or a gigantic carbonaceous chondrite) or comet TSB nature. These can be classified as hypotheses based on the classical concepts of the minor bodies of the Solar System.
The hypotheses of the latter group assume a special type of the nature of TSB, different from asteroids or comets. These include the hypotheses of the antimatter nature of the TSB [54], of the Tunguska meteorite as a microscopic black hole [55], of a "solar energophore" [56] and even of technogeneous origin of the TSB [57-59].
The very fact of the existence of the so diverse views suggests a situation where the phenomenon in question is difficult to explain in all its aspects. Indeed, profound analysis of the factual data on the phenomenon evidences its structural complexity and seeming contradictoriness which restrict its interpretation in traditional terms. It is thus suitable to dwell upon certain most serious difficulties which are to be coped with in any attempt to construct an integrated concept of the Tunguska phenomenon.
1. On the direction of the TSB flight.
2. On some peculiarities of the evidence of eye-witnesses who were close to the Tunguska explosion epicenter.
3. On some specific features of destruction of the forest at the Tunguska explosion epicenter.
4. The energy balance of the Tunguska explosion.
5.On the geophysical effects of the Tunguska catastrophe.