R. Serra, S. Cecchini, M. Galli and G. Longo, EXPERIMENTAL HINTS ON THE FRAGMENTATION OF THE TUNGUSKA COSMIC BODY
Planetary and Space Science, 42, n.9, p. 777-783; 1994

EXPERIMENTAL HINTS ON THE FRAGMENTATION
OF THE TUNGUSKA COSMIC BODY

R. Serra 1, S. Cecchini 2  M. Galli 1 and G. Longo 1-3

1 - Dipartimento di Fisica dell'Universita' di Bologna, Via Irnerio 46, Bologna (Italy)
2 - Istituto di Studio e Tecnologie delle Radiazioni Extraterrestri, CNR, Via De' Castagnoli 1, Bologna (Italy)
3 - Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Via Irnerio 46, Bologna (Italy)

(Published on: Planetary and Space Science, 42, n.9, p. 777-783; 1994)
Abstract
It has been found that in the resin of trees that have survived the Tunguska catastrophe, the quantity of microsized particles trapped at the moment of the event vary markedly from one site to another. The high density of particles found in the trees close to the epicentre, may be an experimental point in favour of the fragmentation model recently developed for the Tunguska body.

Introduction
It has been shown in a previous work, that new experimental data on the composition of the Tunguska Cosmic Body (TCB) can be obtained by searching in the tree-resin for microsized particles trapped at the moment of the event (Longo et al.. 1994). After dating the resin on the basis of the growth tree-rings, the size, morphology and chemical composition of the particles trapped have been determined using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer. Thus, 5854 particles have been found in the resin deposited in the years 1885-1930 on the branches of 7 Tunguska trees. Among them, of particular interest was a group of 518 particles, the so-called "high-Z" particles, containing at least 1 % of one of the following elements: Ti, Cr, Mn, Co, Ni, Cu, Zn, Br, Sr, Ag, Cd, Sn, Sb, Ba, W, Au, Pb and Bi. Indeed, the time distributions of these particles, clearly showing peaks centred on 1908, made it possible to recognise a first list of 14 elements as probable constituents of the TCB.
In the present work, in order to search for experimental indications on the mechanism of the Tunguska explosion, the same particles are examined in more detail taking into account their provenance from trees located in different directions and at different distances from the epicentre of the explosion.

Tunguska trees examined
The region of the Tunguska catastrophe is shown in Fig. 1. As can be seen, the area of overthrown trees extends over 2,150 km2 and inside it about 1,000 km2 of forest has been charred. The "butterfly" shape of the area of forest devastation has been explained by a superposition of a spherical blast wave generated by the terminal explosion of the TCB in the atmosphere and a conical ballistic wave axially symmetrical to the approach path of the body itself (Zotkin and Tsikulin 1966).

 

Figure 1
The region of the Tunguska catastrophe and the trajectory of the TCB.
- . - . -  limit of overthrown trees
------  limit of charred trees
__   hide huts of the closest eyewitnessess.
In the inset:     M = Moscow              T = Tomsk
                          K = Krasnojarsk        V = Vanavara.
The shaded area corresponds to Fig. 3.


The conclusion that the TCB exploded at a height of 8 ± 2 km above a point of the earth surface (60° 53' 09" N, 101° 53' 40" E), usually called "epicentre", has been definitely acquired after more than 20 years of intensive work which mainly consisted in measuring the azimuths of many tens of thousands (out of several tens of millions) of felled tree trunks in 1,475 "points" (with a surface of 0.25 hectare each) in the Tunguska region (Florenskij 1963, Fast 1967, Fast et al. 1967, Fast et al. 1976, Fast et al. 1983). An impressive result of this work is shown in Fig. 2, reproduced from the paper of Fast et al. (1976).

 

Figure 2
Isolines of the field of the azimuths of felled trees, in degrees to the east of the magnetic North (The azimuth = 0° corresponds to the direction of the magnetic North).
The values printed diagonally are the azimuths calculated for 1 km 2 surfaces by the interpolating the measured average azimuth-values.

 

Here the field of the directions of overthrown trees is given in degrees to the east of the magnetic North. In their works Fast and coworkers use a coordinate system whose x-axis (ordinate) and y-axis (abscissa) are rotated to the east of the true meridian respectively by 4° and 94°. In their system the singularity of the field of the azimuths of felled trees is found to have the coordinates x = 39.2 km, y = 20.7 km, which correspond to the geographical coordinates of the epicentre given above. A detailed analysis of the deviations from a strict radial direction of the measured azimuths (taking into account the effect of local relief on the propagation of the explosive wave) made it possible to determine the terminal trajectory of the TCB.
In Fig. 3, which corresponds to the shaded area of Fig. 1, are indicated the itineraries covered by the authors of the present work and the places where the wood samples with resin were taken. As explained in the previous paper (Longo et al. 1994), the resin samples from the roots of tree n° 2, uprooted by the explosion of 1908, were taken as control samples and are of no interest for the present investigation. Nor is tree n° 3 considered here due to the fact that the resin examined was originated mainly by a trauma caused by the 1908 explosion and therefore is different from the other resin samples, taken from withered branches of living trees, which were the main object of searching. This leaves us with 6 trees coming from 4 different sites of the Tunguska region.

 

Figure 3
Itineraries (- - - - -) and places ( O ) where the numbered trees were growing.

 

Results
The 409 high-Z particles, found in the 1902-1914 resin of these 6 trees and which can be related to the Tunguska event, are divided in Table 1 with reference to their provenance from four different sites. Taking into account that the resin surface examined on different trees was not the same, the data are presented in per mil with respect to the total number of particles found in the corresponding trees (fifth column), and in the number of high-Z particles found on one square centimetre of resin surface (sixth column). As can be seen from Table 1, both these values markedly vary from one site to another.
All the resin samples were taken from Siberian spruces (Picea obovata), with abundant resin, except for the samples taken for comparison from tree n° 5, a Siberian pine (Pinus cembra). If the data for trees n° 5 and n° 6 are considered separately, the data for the high-Z particles from Siberian spruce n° 6 reach the values of 331 per mil and 186 particles/cm2. The data on high-Z particles trapped in 1902-1914 in the 5 trees of the same species (Picea obovata) are graphed in Fig. 4.

 

Figure 4
High-Z particles found in the resin samples from Siberian spruces in the period 1902-1914.

 

For each tree, clear abundance peaks of high-Z particles have been found corresponding to the date of the Tunguska event, though with great quantitative difference from one peak to another, as can be seen from the graphs in Fig. 5. This figure shows the main part of the time distribution for the 84 high-Z particles out of 2241 and for the 178 high-Z particles out of 733 respectively found in tree n° 1 and in tree n° 6 in the three periods considered: 1902-1914, which corresponds to particles that can be related to the Tunguska event; 1885-1901 and 1915-1930, which correspond to "background particles" trapped before and after the Tunguska event.
The differences between the data (fifth and sixth column of Table 1) for tree n° 1 and trees n° 4 + n° 8 can probably be related to the different distances of these trees from the epicentre. The position of tree n° 7, on the continuation of the trajectory of the TCB, can explain the enhanced values found in this case.
What remains difficult to understand are the striking differences between the data for tree n° 1 and for trees n° 5 + n° 6 (Table 1) and especially that between trees n° 1 and n° 6 (Figs 4-5). These differences cannot be attributed to the distance from the epicentre on the ground: in each case the distance from the explosion point, at an altitude of about 8 kilometres, is practically the same.

 

Figure 5
Time distribution (per mil) of the high-Z particles found in the resin samples from tree n° 6 and tree n° 1.

 

Discussion and conclusions
The quantitative differences illustrated in Table 1 and in Figs. 4-5 may be fortuitous. Many different causes can affect the quantity of trapped particles: the fluidity of the resin, the screening by other branches or trees, etc... However, the effect of such accidental causes is attenuated for particles coming from an altitude of about 8 km and for data like those of Table 1 referring to tree n° 1 or to trees n° 5 + n° 6, which are each obtained by summing the data for 6 resin samples from 3 different branches. Therefore the different quantity of high-Z particles trapped in these trees is probably too large to be fortuitous.
If this is not so, one possible explanation of the case can be sought in Kulik's idea of the existence of secondary centres of the explosive wave propagation. This idea was based on examination of the results of the aerial photographic survey of the central part of the Tunguska region, carried out in 1938 under the direction of Kulik. The survey consisted in 1,500 good quality photographs, on a scale 1:4700, covering an area of 250 km2 of devastated forest (Kulik 1939, Kulik 1940). As stated by Krinov (1966), "the individual flattened trees were clearly seen in the photographs. Even in the unenlarged prints, the directions in which their tops and roots were facing could be easily made out". These directions, clearly showing the general radial pattern of the treefall, made it possible, according to Kulik, to identify from 2 to 4 secondary centres of wave propagation.
The two main secondary centres, located in the western part of the Southern swamp, are shown in Fig. 6, where the straight lines indicate the directions of the fallen trees. Figure 6 is drawn on the basis of Kulik's figure (1939) with the addition of some lines taken from the subsequent papers (Kulik 1940, Krinov 1949), where the directions of overthrown trees were indicated with the aid of threads extended over a field mosaic photographic chart of the region.

 

Figure 6
The two main secondary centres of explosive wave propagation (circles) according to Kulik (1939, 1940). The straight lines indicate the directions of the fallen trees. The shaded region corresponds to the western part of the Southern swamp (see Fig. 3). The cross shows the position of trees n° 5 and n° 6.

 

It should be noted that Figure 6 does not contradict the trend for the whole Tunguska region shown in Fig. 2. Indeed, taking into account that in the vicinity of the epicentre the homogeneity condition is broken, in Fast's works all the measured azimuth values in a square of 36 km2 around the epicentre are ignored (Fast et al. 1976). The whole area of Fig. 6 lies inside that square, while outside it Kulik's data reveal an essentially radial pattern.
The subsequent direct methodical measurement of the azimuths of fallen trees, begun 20 years after Kulik's aerial survey, has not confirmed the presence of secondary explosive centres (Florenskij et al. 1960, Florenskij 1963, Plekhanov 1964). At that time, however, many details visible on Kulik's photographs had disappeared. The direct measurements, carried out 50-70 years after the catastrophe, refer to conifers only, the fallen broadleaf trees (essentially birches and aspens) having rotted away. On the other hand an accurate examination of the data on the atmospheric and seismic waves associated with the Tunguska event have confirmed that the catastrophe was the result of a single explosion, though not definitely ruling out the possibility of some close explosions, contemporaneous or with short time-delays between them (Pasechnik 1976).
One of the secondary centres of wave propagation, shown in Fig. 6, is located on the ground at less than half a kilometre from trees n° 5 and n° 6. If this centre really existed, it could explain the data, given in Table 1 and in Figs. 4-5, only by assuming that it was at a very low altitude, while the main effects on the whole forest came from an explosion at a greater height. Though it is difficult to justify such an assumption, it seems worthwhile to re-examine Kulik's aerial photographic survey with modern instrumentation and with a computer-aided analysis of the directions of all the fallen trees visible in these photographs. As confirmed to the authors of the present work by N. V. Vasiljev, deputy chairman of the Commission on Meteorites and Cosmic Dust of the Siberian Section of the Russian Academy of Sciences, and by G. V. Andreev, chairman of the Tomsk section of the Astronomo-geodesical Society, Kulik's photographs are preserved in Tomsk in the archives of the Astronomo-geodesical Society and are available for examination.
A second possible explanation of the data is related to the use of the fragmentation model to explain the Tunguska event. Detailed calculations which include the effect of aerodynamic forces that can fracture the meteoroid, and the heating of the bolide due to friction with the atmosphere, have recently been performed, showing that the Tunguska event is fully compatible with the catastrophic disruption of a 60-100 m diameter asteroid of the common stony class (Chyba et al. 1993, Hills and Goda 1993).
In the continuous fragmentation model of Hills and Goda (1993) large asteroids undergo several stages of fragmentation with a progressive increase in the radius of the debris cloud, a decrease in the maximum size of the fragments and a progressive increase in the aerodynamic braking until each piece descends independently to the ground without further fragmentation. Following this model, the energy dissipation ("explosion") at the end of the visible path of the Tunguska impactor is sufficient to explain the 2,000 km2 of forest devastation. Assuming an initial diameter of the impactor equal to about 80 m and an initial impact velocity greater than 20 km/s, the TCB is expected to have produced fragments with a maximum mass of less than 1 kg (less than 1 g for a velocity greater than 30 km/s). This "pile of gravel" would probably have formed a layer less than 1 cm thick with a radius of less than 1 km.
If a great portion of this debris cloud fell in the western part of the Southern swamp, where the Churgima river begins, it was undoubtedly difficult to find "pieces" of the exploded body by beginning to search for them in the whole region of forest devastation more than 20 years after the event.
The high density of microsized particles found in the resin of trees n° 5 and n° 6 may be an experimental point in favour of the fragmentation model. Therefore, it seems worthwhile to continue the search for microremnants of the TCB in the vicinity of the epicentre and, especially, on the southern bank of the western part of the Southern swamp.

Acknowledgment
This research was supported in part by MURST (60 %) grants of the Italian Ministry for the University and the Scientific and Technological Research.

 

Table 1.
High-Z particles found in the resin samples from Tunguska branches in the period 1902-1914. d is the distance of the trees from the epicentre and S the branch-resin surface examined on these trees.

Tree number d
(km)		 S
(cm2)		 High-Z particles Number
per mil		 Particles/cm2
5 + 6 1 2.21 291 268 132
1 3.5 2.12 55 50 26
7 5.5 0.56 37 135 66
4 + 8 7.5 1.45 26 40 18

 

References

Chyba, C. F., P. J. Thomas and K. J. Zahnle, The 1908 Tunguska explosion: atmospheric disruption of a stony asteroid, Nature 361, 40-44, 1993.

Fast, V. G., Statisticheskij analiz parametrov Tungusskogo vyvala, in Problema Tungusskogo meteorita, part 2, 40-61, Izdatelstvo Tomskogo Universiteta, Tomsk, 1967.

Fast, V. G., A. P. Bojarkina and M. V. Baklanov, Razrushenija, vyzvannyje udarnoj volnoj Tungusskogo meteorita, in Problema Tungusskogo meteorita, part 2, 62-104, Izdatelstvo Tomskogo Universiteta, Tomsk, 1967.

Fast, V. G., A. P. Barannik and S. A. Razin, O pole napravlenij povala derevjev v rajone padenija Tungusskogo meteorita, in Voprosy meteoritiki, 39-52, Izdatelstvo Tomskogo Universiteta, Tomsk, 1976.

Fast, V. G., N. P. Fast and N. A. Golenberg, Katalog povala lesa, vyzvannogo Tungusskim meteoritom, in Meteoritnyje i meteornyje issledovanija, 24-74, Nauka, Novosibirsk, 1983.

Florenskij, K. P., B. I. Vronskij, Ju. M. Emeljanov, I. T. Zotkin and O. A. Kirova, Predvaritelnyje rezultaty rabot Tungusskoj meteoritnoj ekspeditsii 1958 g., Meteoritika 19, 103-134, 1960.

Florenskij, K. P., Predvaritelnyje rezultaty Tungusskoj meteoritnoj kompleksnoj ekspeditsii 1961 g., Meteoritika 23, 3-29, 1963.

Hills, J. G. and M. P. Goda, The fragmentation of small asteroids in the atmosphere, Astron. J. 105, 1114-1144, 1993.

Krinov E. L., Tungusskij meteorit, Izdatelstvo Akademii Nauk SSSR, Moscow-Leningrad, 1949.

Krinov E. L., Giant Meteorites, 125-265, Pergamon Press, Oxford, 1966.

Kulik, L. A., Dannyje po Tungusskomu meteoritu k 1939 godu, Doklady Akad. Nauk SSSR 22, n° 8, 520-524, 1939.

Kulik, L. A., Meteoritnaja ekspeditsija na Podkamennuju Tungusku v 1939 g., Doklady Akad. Nauk SSSR 28, n° 7, 597-601, 1940.

Longo, G., R. Serra, S. Cecchini and M. Galli, Search for microremnants of the Tunguska Cosmic Body, Planet. Space Sci., to be published, 1994.

Pasechnik, I. P., Otsenka parametrov vzryva Tungusskogo meteorita po seismicheskim i microbarograficheskim dannym, in Kosmicheskoje veshchestvo na zemle, 24-54, Nauka, Novosibirsk, 1976.

Plekhanov, G. F., Nekotoryje itogi raboty kompleksnoj samodejatel'noj ekspeditsii po izucheniju problemy Tungusskogo meteorita, Meteoritika 24, 170-176, 1964.

Zotkin, I. T. and M. A. Tsikulin, Simulation of the explosion of the Tungus meteorite, Sov. Phys. Dokl. 11, 183-186, 1966.
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