PRELIMINARY RESULTS FROM THE 1961 COMBINED TUNGUSKA METEORITE EXPEDITION
K P Florenskiy
Report read at the Tenth Conference on Meteorites in May 1962
Translated by Spectrum Translation and Research, inc.
Published by Taurus Press, inc., copyright 1965.
INTRODUCTION
Interest in study of unique natural phenomena is, in general, not confined to a single narrowly specialized viewpoint, and the 1908 fall of the Tunguska meteorite is no exception to this. The interests of various scientific disciplines converge on such investigations as at the focus of a mirror, and science itself is spurred to further development whose significance often ranges far beyond the framework of the original specific problem.
In addition to questions pertaining to meteoritics, the fall of the Tunguska meteorite in particular gave impetus to the development of a number of concepts in such areas as the ballistics of supersonic velocities, the theory of large explosions, the structure of comets and features of their chemical composition, geochemical studies of cosmic dust, etc.; furthermore, it encouraged investigation of a variety of natural features in a remote and unexplored but promising area of Siberia. We have the meteorite to thank for the initial investigations of the swamps, soils and forests - studies that are important not only from the scientific standpoint, but also in practical terms.
The conclusions derived from the study of this phenomenon bear on such problems, seemingly unrelated to meteoritics, as the wind resistance of trees in cold soils, improving timber harvests in northern areas, the history of swamp formation, reserves of peat in Central Siberia, etc. The results of this research represent not a random accumulation of data, but merge organically into the integrated whole that is the problem of the Tunguska meteorite.
The history of studies made at the Tunguska meteorite site is divided into three basic stages which set the goals of the project and provided for collection of the necessary factual material without which theoretical consideration of the problem would have been unthinkable. The first stage, begun by L. A. Kulik, called for the collection of preliminary information, eyewitness accounts and data from observer stations in order to pin point the area of impact and to describe the physical phenomena that occurred outside the impact site. The second stage, also linked with the name of Kulik, involved a thorough study of a small area of ground in which a number of funnel-shaped depressions had been discovered; these had erroneously been identified as meteorite craters. The third stage was undertaken in 1938 with an aerial-photographic survey of a considerable area of the terrain, but essentially this stage did not develop fully until the author's 1958 expedition, which made an integrated study of the site and of all the possible consequences of the fall.
The 1958 expedition [1] demonstrated the lack of justification for classifying the Tunguska fall as a crater-forming type and established that the center of the meteorite shock wave was located at some height above the ground; in addition, the expedition provided the first general map of the destruction that had been wrought, collected specimens of extraterrestrial dust from the area of the fall, drew attention to peculiarities of tree growth after the catastrophe and pointed to the possibility of using biological indicators; finally, the expedition pointed up the need for a careful study of the forest fire, etc.
As a result of the work accomplished in this stage, the earlier theoretical concept of the fall was subjected to substantive review. A program for further study of the Tunguska fall was proposed, accepted by the Committee on Meteorites of the USSR Academy of Sciences, and subsequently approved at the Ninth Conference on Meteorites. The expedition set for1961 made provision for a further series of investigations to determine details of the meteorite's effects and called for an expanded search for fragments of meteoritic matter.
The great popular interest in the Tunguska meteorite that arose after the 1958 expedition was to a considerable extent fostered by the fantastic suggestion of a nuclear origin for the explosion, an idea based on factual material of questionable competence. While I am aware of the advantages of sensational publicity in drawing public attention to a problem, it should be stressed that unhealthy interest aroused as a result of distorted facts and misinformation should never be used as a basis for the furtherance of scientific knowledge.
A number of independent groups left for the area of the Tunguska meteorite fall in 1959 and 1960. Of these, the most serious was that of G.F. Plekhanov, whose work in 1960 was partly financed by the Siberian branch of the USSR Academy of Sciences, with support from the Committee on Meteorites. Basically, the project followed a plan that had been prepared in 1958; a substantial volume of factual material was accumulated. At the same time, it would not be out of place to point out that a number of projects carried out in 1959 and 1960 were based on fantastic ideas, and this resulted in unproductive expenditures related to the execution of the projects themselves, as well as in establishment of the fact that they had no relation whatever to the problem of the Tunguska meteorite.
In this connection, we should make particular mention of A.V. Zolotov's group, which worked in this area in 1959 and 1960 and submitted hasty and unfounded conclusions on the basis of extremely inadequate and random data (see the resolution of the Ninth Conference on Meteorites. Meteoritika, Volume XX, 1961).
Thus at the time when the 1961 expedition began its work, the situation was basically as follows:
1. On the basis of visual observation the general contour of the radially flattened forest area identified by Kulik was mapped out. Three zones can be distinguished within the limits of this area: random flattening (standing timber), mass flattening and partial flattening in a specific direction; the general plotting of these zones was confirmed in 1960. However, the boundaries of the zones (particularly to the northeast) and the direction of flattening call for additional refinement and quantitative characterization. There was no search for isolated islands of fallen trees outside the area of general destruction. Approximately 40 control plots, each 0.25 hectare in area, were laid out to obtain quantitative characteristics of the destruction [1, 2].
2. There are no traces of a powerful ground meteorite explosion in the area; this is confirmed by a study of the south morass, where this fact was established through a number of helological studies, profiling of the bottom [3] and three magnetometric profiles [2]. The thermokarstic funnels [4] are not directly related to the explosion of the meteorite [l, 3, 5] and no magnetic anomalies were observed [2]. The possible stimulation of thermokarst development as a result of the fall [4] calls for additional helological research, as well as identification of the 1908 peat layer for purposes of a stratigraphic hunt for meteoritic matter.
3. The presence of live trees at the center of the catastrophe [1, 2, 4, 5] bears witness to the comparatively low level of any possible flash burning, whose general nature, along with the causes of the forest fire of 1908, requires additional study.
4. The large-scale map prepared from the results of the 1938 aerial survey of the center of the catastrophe by the MIIGAIK [Moscow Institute of Geodetic, Aerial Survey and Cartographic Engineers] on assignment from the Committee on Meteorites requires considerable revision based on field work. This map also confirms the existence of many stands of trees that survived in the central section of the affected area.
5. The soil study made in 1958 [1, 6] shows that the magnetite and silicate spheres first observed by L. A. Kulik and then by A. A. Yavnel' occur in small concentrations in the impact area. At the moment they represent the only possible fragments of the Tunguska meteorite and call for further study.
Geochemical (metallometric) attempts to identify zones enriched with meteoritic matter [l, 2] produced no clear results and are of doubtful promise, unless the objects of the study are subjected to preliminary concentration .
6. The features of accelerated tree growth established in 1958 [1] have been confirmed by a great volume of data [7] and are peculiar to the central region of the impact area. In view of the fact that the causes of this phenomenon are not clear, work along this line should be continued in order to seek out biological indicators characterizing features of the 1908 fall.
7. On the basis of available data, it is at the present time held most likely that the meteorite "exploded" in the air. By way of explanation of this phenomenon a number of mechanisms have been proposed [8, 9, 10, 11, 12], but these cannot be regarded as reliable because of the inadequacy of working data.
The most probable hypothesis is that which ascribes a cometary origin to the meteorite [13, 14], whose structure was friable [1, 8, 15, 16]. Available data are inadequate to form a clear picture of the phenomenon.
Organization of the project. The expedition of 1961 was organized on the initiative of the Committee on Meteorites on the basis of a decision taken by the Ninth Conference on Meteorites and the 30 September 1960 Decree of the Presidium of the USSR Academy of Sciences [17]. In this connection, we should give special mention to Doctor of Geological and Mineralogical Sciences Ye.L. Krinov for his major organizational role as Scientific Secretary of the Committee on Meteorites and his monumental scientific contribution to the study of the Tunguska meteorite. The entire problem received the untiring attention of Academician V.G. Fesenkov, who directed the astronomical and physical investigations, and Academician A.P. Vinogradov, who was in charge of the work associated with the material composition of the meteorite.
The nucleus of the expedition was made up of members of the Committee on Meteorites and the Vernadskiy Institute of Geochemistry and Analytic Chemistry; however, individuals from other organizations also participated (The Soil Institute, The Main Botanical Garden, Moscow State University, etc.). The Helology Section of the Forestry and Lumber Institute of the Siberian branch of the Academy of Sciences and the Forestry Assessment Section of the All-Union Forest Aerial Photographic Survey Association of the Main Administration of Forestry, USSR Ministry of Agriculture ("Lesproyekt") demonstrated interest by participating directly in the expedition. An outstanding feature of the expedition was the enlistment of a large contingent organized with the support of the Tomsk branch of the All-Union Geographic Society into the Combined Autonomous Expedition, designated KSE, of whose work we spoke earlier. This group joined the ranks of the 1961 expedition and worked under its authority. G. F. Plekhanov, the elected leader of this group and both a physician and an engineer, was put in charge of a section of the expedition; the numerical strength of the entire group considerably exceeded the original table for the expedition. In terms of qualifications and conscientious attitude, the Combined Tunguska Meteorite Expedition participants in the Combined Autonomous Expedition were capable of carrying out independent assignments unrelated to the responsibilities that they had assumed within the expedition.
We should stress that proper and well-organized scientific leadership of such projects is of particularly great importance for the required concentration of research on major problems.
The number of persons participating in the work of the meteorite expedition reached a total of 80, of whom some did not stay out the season. The program of the expedition called for extensive use of a helicopter to deploy individual groups and to transport heavy specimens. The actual unavailability of a helicopter, however, compelled the members of the expedition to move on foot; this significantly reduced both the total number and the size of the specimens collected. Some of the equipment and supplies even had to be dropped by airplane to the expedition base at Kulik's clearing.
Part of the expedition departed from Moscow on 10 June, arrived at Vanavara on 16 June, and immediately began its work, gradually reaching full strength with the arrival of other colleagues. The expedition's works schedule was thrown off by the lack of the helicopter. There were no communications with Vanavara. The greater part of the expedition returned to Vanavara around the first of October, with the first snowfall. During the next few days the Chamba and other slow-flowing rivers began to freeze. The logistic-support group of the expedition arrived in Moscow on 24 October, having successfully completed the evacuation of all expedition equipment (I.N. Yeliseyev, Ye.I. Malinkin).
I. STUDY OF THE EFFECTS OF THE METEORITE FALL
FOREST FLATTENING
The flattening of the forest most accurately reflects the passage of the destructive wave accompanying the fall of the 1908 meteorite and can serve as a physical characteristic of the phenomenon.
Fig. 1. Dynamometric testing of tree resistance
A study of the devastated forest showed that the region of the forest flattened in 1908 was not one of homogeneous primeval intact taiga, but had had a complex history that must be taken into consideration in interpreting the data. Thus fire expert N.P. Kurbatskiy (The Forestry and Lumber Institute of the Siberian Branch of the Academy of Sciences) draws the following conclusion: " ...the region of meteorite impact in 1908 was basically a fire-devastated area that had been subjected to a treetop fire during the first half of the last century. A partly flattened dead and rotting forest was standing in this area. New forest growth had appeared among the dry and charred trees. By 1908, this second growth was some 70 to 100 years old. The southern and southeastern areas had apparently been subjected to fire somewhat earlier in the past than the central basin. The dead-forest was first flattened, and a fire then swept this territory. It is not out of the question that the set two occurrences took place at the same time... The fire did not destroy the trunks of living trees, but only scorched conifer needles and small twigs. The forest destroyed by the fire of 1908 was not flattened at that time but has in large part remained standing in the form of dead timber to the present time. The old stand of dead and badly rotted timber left by the fire of the last century could have been laid low during the fire itself and been scorched from the bottom as a result of a ground fire... There is nothing out of the ordinary in the suggestion that the larches were able to stand dead in the area of the meteorite fall for a period of 70 years after the fire of the previous century."
On the basis of the forestry-assessment records, V.G. Berezhnoy (All-Union Lumbering Office of the USSR Ministry of Forestry - "Lesoproyekt") draws the following preliminary conclusions: "...it has been shown that at the time of the catastrophe the stand of trees in the area had, to some extent, been weakened by the fire at the end of the last century. For example, the great fire that produced the so-called 'western flattening' occurred in 1896. Larger diametral accretions are found in the area over which this fire had passed. The northern and eastern parts of this area (and quite possibly also the center) had been subjected to a fire during the 1880's... There are stretches of practically 'unburnable' forest within this area. These sections did not burn during the fire." At the same time, it is obvious that an estimate of the force of the shock wave that is based on the number of flattened trees must necessarily take into consideration the condition of the forest at that time. The wind resistance of a comparatively recent stand of dead timber is greatly increased as a result of reduced area catching the wind, particularly if the root system is still strong; later it diminishes to zero as the trees and their roots decay. Dynamometry studies of the flattening (wind resistance of the trees) in this area by means of a winch and a dynamometer (Fig. 1) (K.A. Lyubarskiy, I.T. Zotkin) yielded the following preliminary results (95 trees were studied): "There is no relationship between the felling moment and the species and age of the tree. There is a distinct relationship between the moment and the tree diameter, analytically nicely described by a parabola (Fig.2). These parabolas are completely identical for fine (melkozem) and rocky soils. For moist riverside soil the parabola takes a noticeably flatter course (the felling moments are significantly smaller). The relative scatter diminishes in inverse proportion to the diameter of the tree. It is not related to the azimuth of the felling or to the direction of the slope (upward, downward, to the side). From the standpoint of force there is no difference between snapping and uprooting, i.e., both cases array themselves along the same parabola. The parabolic relationship is retained for dead-timber logs, but the parabola in this case is extremely flat, the felling moments are minimal, and the scattering is therefore very wide" (the dead-timber data pertain to timber killed in 1908, Table 1).
TABLE 1
Average Felling Moments for Trees in the Area of the Tunguska Catastrophe (Pines, Larches) (Preliminary Data)
|
Area no. |
Nature of area |
Tree diameter, cm | |||
|
15 |
20 |
25 |
30 | ||
|
|
|
Felling moment, ton-m | |||
|
1 and 2 3 4 |
Dry soil Marshy soil Brush dead 53 years |
1.1 0.7 0.4 |
2.3 1.5 0.8 |
3.7 2.8 1.3 |
5.5 4.2 1.9 |
Unfortunately, we were unable to study the relationship between log strength and age - a matter of unquestionable interest - because of the difficulties which we encountered in determining the time at which this stand of timber was killed off.
Fig. 2. Felling moment as a function of tree diameter.
1) Trees in fine and rocky soil; 2) dead-timber stand of 1908.
That there is no relationship between the strength of a tree and its species would seem at first glance to be a rather bold conclusion: a definite distinction is made in forestry between "wind-susceptible" and "wind-resistant" species. Apparently this classification is not applicable to regions of cold and frozen soils, where deep root systems ("tap roots") cannot develop and where the roots of virtually all species develop in similar fashion, along the surface of the ground ("shield" or lateral root system), in virtual independence of the nature of the soil, i.e., the slow thawing of the soils in the summer limits the depth of root penetration.
The soil and soil-temperature variation studies conducted in the area by soil scientist A.A. Yerokhina (Dokuchayev Soil Institute of the USSR Academy of Sciences) demonstrated that there is virtually no permafrost to a depth of 1.5 to 2 meters in the dry forest soils of this area, but that the seasonal frost thaws out only toward the beginning of August. The moist and marshy soils have permafrost base layers. The general characteristics of the felling were studied in a number of ways.
Fig. 3. Distribution of flattened trees by direction in various sample areas.
A histogram of this type, including about 100 trees, corresponds to each of the arrows in Fig. 4.
To provide an over-all description of the flattening effect, we used a graphic method (V.G. Fast and D.V. Demin) which involved taking the azimuths of all of the fallen trunks in a sampling area containing some100 flattened trees (generally 0.25 hectare) and plotting their variation diagram (Fig. 3) with indication of the trunk thicknesses. The numbers of dead-timber and old living trees were also noted. As a result of the work carried out in 1960 (Plekhanov et al.) and 1961, approximately 200 such areas were investigated to obtain a reliable profile of the entire area of flattened forest. Both the mean direction of the flattening, which reflects the direction of the wave front, as well as the directional dispersion of the flattened trees, which is a function of shock-wave force, can be used to describe the motion of the wave. The evaluation yielded a somewhat exaggerated value, since areas with the most clearly defined flattening were selected as controls and, moreover, it was impossible to take into consideration the relationship between the trees that had been felled live and those that were dead.
The total area of flattened forest covers approximately 2000 square kilometers and generally coincides with the boundaries established by the1958 expedition. However, it has now been established that the flattening extended farther to the northeast, along the Sil'gami Range to the summer trail leading to Strelka, a fact which had also been noted in 1960. Thus the general contour of the felled forest takes the form of a triangle, apex forward, symmetrical about the meteorite trajectory determined by Krinov. It is characteristic that compilation of a large volume of data shows no significant deviations from the radial in the directions of the fallen trees. The arrows obtained by this method are more or less uniformly distributed over the entire felled area (Fig. 4).
Fig. 4. Flattening of timber on the basis of 1961 data.
1) Sampling areas encompassing 100 trees; 2) directions based on several trees; 3) 1960 routes; 4) stretches with limited or poorly defined flattening; 5) stretches in which there was no flattening at all; 6) positions of the forestry-assessment plots; 7) huts; 8) boundaries of the 1908 fire where its natural propagation is seen distinctly; 9) general boundary of flattening; 10) southeastward variant of the meteorite trajectory, after Krinov.
Figs. 5. Present appearance of felled timber.
For purposes of evaluating the information from the control areas and in order to establish a detailed picture of the forest destruction, special forestry-assessment plots were surveyed and both the living and dead tree son these areas described (Fig. 5). Over a number of years, beginning with 1958, the methods employed for this part of the operation were gradually improved and in 1961, in its final form, the work came under the direction of V.I. Nekrasov (Main Botanical Garden of the USSR Academy of Sciences), V.G. Berezhnoy and G.I. Drapkina (All-Union Forest Aerial Photographic Survey Association of the Main Administration of Forestry of the USSR Ministry of Agriculture). Some of the forestry-assessment plots were staked out and partly described in 1960 (V.I. Nekrasov, Kolesnikov); they were distributed in the form of a cross passing through the epicenter of the destruction to the boundaries of the flattened forest (55 plots), with an additional 40 plots located in the intervening quadrants. The relationship between the percentage of fallen trees and their directional distribution can be established on the basis of the control areas, which can also be used to describe the general condition of the forest in 1908 and at the present time.
A special investigation was undertaken (Zotkinetal.) to detail the felling in the center of the area covered by the chart prepared by the Moscow Institute of Geodetic, Aerial Survey, and Cartographic Engineers. This study involved the plotting of arrows indicating the direction of the fallen trees at points not examined by aerial photography, and surveys of special control areas to ascertain the effect of local relief on the action of destructive wave. This results in rather convincing evidence that there is a pronounced increase in directional flattening on reverse slopes (with respect to the epicenter) near the epicenter-an indication of an "explosion" at altitude.
As we have noted, the "zone of indifference," also known as the "chaotic flattening" or "telegraph pole" zone, occupies an area somewhat smaller than that indicated by the 1958 chart. This may indicate a somewhat lower center of wave origin than had earlier been supposed (Fig. 6).
Thus the collected material on the flattening of the timber permits us to draw the following conclusions:
1. The total area of flattened forest is in rather good agreement with the map prepared in 1958. An exception is the extension of the flattened area to the northeast, as can be seen from the map showing the propagation of new forest.
2. The collected material is quite complete and there would be little gained from further field refinements prior to complete evaluation of the data obtained.
3. Nowhere in the directional studies of the fallen trees were any significant deviations from radial flattening noted.
4. By 1908 the forest consisted to a considerable degree of standing dead timber, and computation of both the over-all force of the shock wave and its isobars must be accomplished with consideration of this factor. Areas of forest that had not burned by 1908 survived to a significant extent and were not flattened.
5. The influence of local relief on timber flattening in the vicinity of the epicenter is of a nature such as to confirm that the explosion took place at some height.
6. The slightly smaller central zone of random dead-stand flattening as compared with the 1958 data speaks against an excessively high explosion center.
7. The data collected must be subjected to careful evaluation.
THE FOREST FIRE OF 1908 AND FLASH BURNING OF THE TREES
As had been pointed out in 1958, a forest fire originated at the point of meteorite impact and spread in the usual manner. Thus the boundaries of the fire do not coincide with the boundaries of a possible flash fire, as Zolotov sought to prove (1959). [* A report on the work of A.V. Zolotov during his 1959 expedition.] While no one has dismissed the possibility of a flash fire, it has not been definitely established either. The 1961 investigations of the injuries inflicted on live trees in the vicinity of the epicenter (Zenkin et al.) revealed a large number of still surviving trees and the fact that the injuries to these trees were oriented in nature.
Fig. 6. Schematic map show in a distribution of flattening, dead timber and live trees at the epicenter.
1) Flattened trees; 2) dead timber; 3) surviving trees; 4) expedition shelters.
N.P. Kurbatskiy, a member of the 1961 expedition, describes the features of the fire in this area as follows: "...distinct signs of the spreading of the 1908 treetop fire were found in the form of arcuate strips and surviving older forest to the north of the Kimchu River, at a distance of 1 to 2 km from the bank, in the stretch from Lake Cheko to the extensive marshes on the left side of the river. In this area the fire spread from south to north. Indications of the fire's spread were found on heights to the north of Lake Cheko, as well as on the western slopes of hills 373.6, 491.0 and 476.0. From here the boundary of the fire area can be traced easily along an arc from the mouth of the Chavidokon River to Mount Shakharma. The surviving traces of the fire-line advance indicate that it spread toward the west and southwest, i.e., as if from a central depression. The treetop fire here gradually changed into a ground fire, with some damage to the old forest. On the northeastern slopes of the heights where the headwaters of the Churgim River rise there are arcuate strips of old forest, convex toward the south. We find identical signs of the spreading fire on Mount Shakharma, but facing east... The old forest remained intact only in narrow strips along the banks of rivers with highly developed valleys and in the form of solitary trees in the midst of swamps and rock streams."
"The rounded shape of the fire site and the complete destruction by fire of the old forest over an extremely great area are outstanding features of the area; it differs in these respects from ordinary forest burnouts after the passage of treetop fires in the presence of a strong wind... Of the surviving areas of primeval taiga, we inspected only two, 2 to 5 hectares in area, situated on the flat saddle slopes of the northwestern heights surrounding the central depression... During the course of this inspection on 28 June 1961, the sixth day after a heavy rainfall, the ground vegetation in these areas was extremely wet and would not burn, although flammable material on the ground ignited easily throughout the rest of the impact area. It is obvious that these areas differ from the remaining territory in having an elevated resistance to fire. In the past (apparently, in 1908) a ground fire penetrated these sections from the fringe growth of that time, but did not spread through them."
"The 1908 fire flared up at several points: in the central basin, on the territory adjacent to the Khushmo River between the Churgim and Ukagitrivers, as well as on the northeastern slopes of the Khladnyy Range. The fire was preceded by a dry spell, which was responsible for the widespread and uniform burning of the forest and the spreading of the fire to the marshes. Such a set of circumstances is possible for this area in the month of June, but not altogether usual. The 1908 fire spread rapidly through the treetops of the meteorite area, before a wind moving at a speed of 6 to 10 m/sec. At this velocity, the wind was a local phenomenon and a result of powerful rising currents of combustion products and heated air. The fire lasted for at least five days. During this time the direction and velocity of the wind varied with the diurnal variations in temperature and with changes in the intensity of the fire front as it enveloped various elements of the terrain relief. The fire died out during the first ten days of the month of July in 1908 as a result of unfavorable weather conditions. The trunks of live trees did not burn during the fire; only needles and small twigs were scorched."
As we are well aware, forest fires generally start on ignition of floor litter, which is the most highly combustible material. To determine the fire hazard in this area and the quantity of heat required to start a fire, a series of experiments were carried out to ascertain the moisture content of the various types of ground litter as a function of the length of a dry spell (N.P. Kurbatskiy, T.M. Sleta). Evaluation of these data by the Forest Fire Prevention Laboratory at the Forestry and Lumber Institute of the Siberian Branch of the Academy of Sciences should make it possible to establish a numerical scale for the minimum quantity of radiant energy required to start the 1908 fire.
The maximum quantity of energy can be found on the basis of the fact that various species of trees (larches, pines, and cedars) survived in the area of the epicenter, in some cases even retaining live branches.
Fig. 7. Physiological scorching of larch twig near epicenter by light. The oriented cambium injury of 1908 can be seen; the twig at that time was no more than 8 mm thick. The injury was subsequently completely invested.
Studies of live branches (older than 53 years) on trees growing in open areas were undertaken to seek out evidence of a flash fire, to evaluate its intensity and direction (G.M. Zenkin, A.G. Il'in, L.F. Shikalov et al.); in part, this is a continuation of work started in 1960 (Plekhanov et al.).
It was established in these investigations that branches (primarily of larches) which in 1908 did not exceed 8 to 15 mm in thickness often show traces of injuries that date from 1908, and have subsequently been invested by bark (Fig. 7). The damage is noticeable on the upper portions of the branches, thus making it impossible to associate these injuries with ordinary fires. Moreover, they are oriented toward the supposed center of the meteorite explosion. The severity of these injuries diminishes significantly with increasing distance from the epicenter. The damage indicates a fungus disease that affected the injured parts of the cambium. We thus have a serious basis for assuming that these injuries are a result of physiological scorching of cambium cells during the fall of the meteorite.
Statistical reduction of the collected material will assist us in establishing the factors responsible for the appearance of these injuries, and, if they are associated with a flashfire, will be helpful in determining its intensity and direction. The latter is of interest from the standpoint of determining the spatial position of the meteorite at the instant of maximum brightness. [* G.M. Zenkin places the emission center at 1.5 km to the southeast of the epicenter of destruction, at a height of about 5 km.]
TABLE 2
Injury Distribution (Flash Fire?) at Various Distances from Epicenter*
|
Direction |
Distance from epicenter, km | ||||||
|
3 |
4 |
5 |
6 |
7 |
8 |
9 | |
| North ……………….
East ………………. South ………………. West ………………. |
- - + (1) + (1) |
+ (2) + (2) + (1) - |
- - + (2) - |
- + (2) - + (1) |
- - + (1) - |
- - (1) - - |
- (3) - - - (1) |
The + sign indicates injury observed; the - sign indicates no injury observed; (3) represents the number of trees inspected.
The distribution of the above injuries as a function of distance from the epicenter, after the data of Zenkin and Il'in, is presented in Table 2.
Shikalov and Ivanova submit data indicating that analogous instances of injury were observed in less pronounced form at distances up to 9 km to the north and west of the epicenter, and as far as 10 km to the south. Extremely rough and approximate calculations of the energy required to produce physiological scorching and local mortification of the cambium on a branch approximately 1 cm thick yield a value of 5 to 15 calories per square centimeter. The illumination value cannot be significantly higher (more than double this figure), since this would lead to marked charring of the bark, and no such phenomenon was observed. Approximately the same energy is required to ignite dry forest debris, and this could lead to a forest fire.
According to a preliminary evaluation, this result for the energy of the light pulse (at a distance of up to 9 km from the epicenter) is smaller by approximately half an order than the result obtained by Zolotov.
For purposes of comparison we cite certain data (Table 3) on flash burns suffered in atomic explosions (see "The Effects of Nuclear Weapons," [Deystviya yadernogo oruzhiya], translated from the English, Voyengiz,1960)
TABLE 3
Luminous Irradiation Resulting in Injury, for Two Explosion Forces
|
Effect |
Energy of irradiation by light, cal/cm2 for TNT equivalent of | |
|
20 kilotons |
10 megatons | |
| Burns, human skin:
First degree Second degree Third degree Charring of fir, pine and maple bark Kindling: Dry rotten wood Fine grasses Fallen leaves White-pine needles |
- 2 5 7 10 - 4 5 6 6 |
- 3.5 7 11 15 - 9 10 12 14 |
STUDY OF THE EFFECT OF THE METEORITE ON THE SWAMPS
The observations of the 1958 expedition indicated that there was no meteor crater in the south morass and that there was no relationship between the formation of the thermokarstic funnels and the fall of the meteorite; however, these observations were not sufficiently reliable. Yu.A. L'vov, Kovalevskiy et al. undertook a rather detailed inspection of the swamps in 1960. They established that "all of the structural features of the basin's marshes are readily explained by terrestrial factors. No interbedding of peat and soil in the south morass was observed, Kulik's data not withstanding... The fall (B.I. Vronskiy) of small meteoritic fragments cannot be used to explain the thermokarsts...a four-to-six-centimeter peat layer in which the 1908 stratum was deposited is easily identified... Even in the event of negative results, the search for meteoritic matter in the peat must be repeated" (taken from a letter by L'vov to Florenskiy, dated 28 December 1960).
The section of the helology team from the Forestry and Lumber Institute of the Siberian Branch of the Academy of Sciences, headed by Doctor of Biological Sciences N.I. P'yavchenko, worked from 22 June through 6 July 1961, while the main unit of the helological section headed by S.P. Yefremov remained on the job to study the surrounding marshes throughout the entire summer.
Fig. 8. A view of the south morass.
I cannot dwell here on a discussion of the extensive work program undertaken by this unit, even though it is of considerable interest to the helologist. N.I. P'yavchenko formulates his replies to questions concerning study of the Tunguska fall in the following manner.
"Differentiation of the south and north morass surfaces into hillocks and 'mochazhina' [land permanently wet from outflow of underground water] is a result of thermokarstic processes and, possibly, a result of phenomena related to ancient water erosion... The formation of funnels in the swamps is not associated with the fall of the meteorite or fragments of it. No embankments of ejected peat have been built up around the funnels, the peat layer is not intermixed with the soil and it is of rather along the line of contact with the peat hillocks are steep and frozen... Numerous core samples of the peat deposit uniform depth throughout the entire area of the funnel, the mineral base is flat, and the walls in the south and north morasses fail to reveal any mixing of the peat with the 'ooze' or subsoil. The peat deposit exhibited rather clearly oriented stratification throughout... The 'embankments' on the surface of the south morass, which extend, with slight separation from one another, in the direction perpendicular to the runoff, are ordinary ridges 20 to 30 centimeters in height covered with dwarf arctic birch, bog underbrush, hypnum and sphagnum, and quite often with woody vegetation (Fig. 8). These ridges are formed in wet marshes because of surface cracking due to frost and raising of the peat around the edges of the cracks, which improves the drainage of the area adjacent to the crack, thus favoring the growth of vegetation requiring less moisture... Traces of fire stand out clearly in the peat deposits of the hummock areas in the north and south morasses. In the majority of cases two fire levels stand out clearly: an upper level approximately 20 and in spots 30 years old; a lower level, about 50 years old... The lower level is quite thick, ranging from 3 to 5 cm, and occasionally more. This level contains much charcoal, ashes and plant residue... All of the old trees that survived on the peat hillocks show searing only on the lower parts of their trunks, which indicates that the fire moved along the ground; this is natural in view of the high degree of thinning of woody growth in peat bogs... No regular intensification in the growth of trees, dwarf arctic birches, bog underbrush, and mosses in the marshes after 1908 was observed. Among the old larches, the increment during the past 2 to 3 decades has been insignificant. Young larches and pines appearing on the peat hummocks after 1908 are characterized by growth satisfactory for frozen peat bogs and rather uniform height and diameter increments. In addition, severely stunted trees are encountered. These differences are associated with the ecological conditions of vegetation growth... The increment in sphagnum in the permanently wet ground and thermokarsts amounts to 2.4-2.5 cm annually, which is quite normal for this zone. Under extremely dry conditions Sphagnum fuscum increases at a rate of about 1 cm per year on the peat hummocks."
There were no clearly defined changes in the hydrological regime of the marshes that were associated with the events of 1908. The gradual aggrandizement of the morass at its boundaries and the partial submergence of dead growth is typical of many of the marshy areas of this region, bearing absolutely no relation to the fall of the meteorite. The age of the south morass runs to many thousands of years, and determination of the absolute age at which swamp formation began is a matter of considerable interest, since it represents information totally unknown with respect to the marshes of Central Siberia. This determination can be accomplished by using the radiocarbon method on specimens of wood raised from the very bottom of the south morass.
Thus as a result of detailed helological investigations in 1960 and 1961 we may regard as quite reliable the conclusion that there is no relationship between the fall of the meteorite in 1908 and the manner in which the south and north morasses were formed.
THE BIOLOGICAL EFFECT OF THE FALL
The 1958 expedition drew attention to the pronounced change in the rate of growth of a number of trees subsequent to 1908, and pointed up the possibility of using "biological indicators to ascertain features dating from 1908, as well as general changes in conditions brought about by the fall of the meteorite." Yemel'yanov and Nekrasov showed in their subsequent work that accelerated growth is characteristic of a wide region around the center of the meteorite fall. A number of biological indicators suggested the probability that this phenomenon was in some manner associated with meteoritic matter and should be studied. In 1960, together with a number of other colleagues, they began laying out the forestry-assessment plots which made it possible to bring to light the unusual nature of this phenomenon.
A group of biologists joined the 1961 expedition of the Committee on Meteorites to establish the boundaries and causes of the accelerated growth of forest in this area. Because the problem of accelerated forest growth and increased timber harvests in the areas of the north is of prime practical importance, the All-Union Forest Aerial Photographic Survey Association of the Main Administration of Forestry of the USSR Ministry of Agriculture participated in the project by dispatching a forestry-assessment section to work together with the expedition. This work was performed by Candidate of Biological Sciences Nekrasov, a forestry specialist; by Berezhnoy, Drapkina, and a number of others. A number of important details incidental to this work were brought to light in connection with the nature of the meteorite-connected injuries sustained by the forest. As a result of the work carried out in 1960 and 1961, a total of 95 forestry-assessment areas were laid out, and all of the biological aspects of the phenomenon were studied here.
The change in the rate of forest growth can be associated either with a change from the normal ecological conditions of growth due to the tremendous forest fire and flattening of the forest which took place in 1908, or with the action of stimulants that appeared in the soil on disintegration of an extraterrestrial body of unknown composition.
Although literature sources indicate that the aftereffects of ordinary forest fires and forest uprooting with which we are familiar from European silviculture should not last longer than 15 to 20 years, they persist, occasionally without noticeable abatement, for a period of 40 to 50 years, in the area of the meteorite fall. Nekrasov has expressed doubt as to the possibility of explaining this phenomenon in terms of purely ecological factors (more sunlight as a result of thinning, the effect of ash fertilizers, a change in the heat and moisture regime of the soil, soil aeration, etc.).
The study of the soil heat regime which was conducted by Soil Scientist A.A. Yerokhina to shed light on the influence exerted by frost phenomena demonstrated that there was no evidence of permafrost in the dry forest soils to a depth of at least 1.5 to 2.0 meters, and that it is difficult to detect any change in the thermal regime of the soils with the methods available to the soil scientist.
To supplement the silvicultural work, the growth rates of oats in the soils of various sections of the region were determined (A.B. Osharov, a plant physiologistat Tomsk State University); soil microflora were identified (N. V. Vasil'yev, a microbiologist from the Tomsk Medical Institute), and a study of the root systems of trees and their changes subsequent to 1908 was carried out (Osharov). The possibility of detecting a change in the thermal regime of the soil and the related possible deeper penetration of the root systems after 1908 prompted this last project. Particular attention was devoted to the growth pattern of bog vegetation communities during the helological studies.
The office evaluation of the secured data, which should supply the answers to these questions, has not been completed, and thus the expedition has not as yet formed any final conclusion regarding the results of the project. However, we can make the following preliminary comments.
1. Where we have a similar combination of fire and forest flattening due to ordinary causes outside the environs of the epicenter of meteorite impact ("the western flattening" of 1896, the fire in the vicinity of the Chamba in 1937), an equally persistent acceleration of forest growth is encountered.
2. There is no indication whatever of any change in the growth rate of bog communities beyond the moss layer which grew directly on the mineral-rich 1908 layer over which the fire had passed.
3. Although acceleration of forest growth after 1908 was observed in areas bearing no traces of the fire or flattening of the forest, these areas are situated in the immediate vicinity of extensively destroyed areas and are spatially limited, and thus subject to the over-all regime of a much wider region.
4. No clear differences corresponding to boundaries of accelerated forest growth were ascertained as a result of cultivation of oats in various soil areas of the region in which the meteorite fell.
5. The boundaries of accelerated forest growth follow the boundaries of the fire and flattening of the forest quite closely, but they do not correspond to the distribution of meteoritic matter as obtained by other methods.
All of the above compels us to express grave doubt as to the possible participation of meteoritic matter (in one form or another) in the stimulation of tree growth in this region, and suggests that the prolonged aftereffects are associated with features of the ecological conditions of forest growth in this zone of Central Siberia. Forestry experts Berezhnoy and Drapkina (All-Union Forest Aerial Photographic Survey Association of the Main Administration of Forestry of the USSR Ministry of Agriculture) came to the same conclusion. This has no bearing on the possible practical application of the features studied to the promotion of silviculture in the northern regions of Siberia.
II THE SEARCH FOR METEORITIC MATTER
Various dispersion states must have been present during the flight and disintegration of the Tunguska meteoric body in the atmosphere, differing in the nature of their scattering over the surface of the earth in accordance with time of formation and degree of dispersion. At the very least, we may assume the presence of the following forms to have been probable.
1. The dust-and-gas tail of a comet, torn away in the uppermost layers of the atmosphere and dispersing over the entire surface of the earth to produce bright nights.
2. The dissipation of ionized gas in the atmosphere along the flight path of the meteorite, without any settling of meteoritic matter.
3. Liquid and vapor condensation products swept from the heated substance of the meteorite to form its train. Coarse fractions of the train may have reached the surface of the earth in the form of molten droplets that froze into meteoritic spheres, settling along a more or less clearly defined strip, the finer droplets scattering uniformly or settling out over a great distance.
4. The "explosion" products of the meteorite. Their state depends primarily on the composition of the meteoroid. Volatile cometary components dissipate without leaving a trace; the remaining components form local conglomerations depending on the nature of the explosion and the degree of dispersion. Individual large meteoritic fragments may reach the surface of the earth along the line of the meteorite's flight path, beneath its train, or at a point beyond the epicenter of the "explosion." There cannot be many such fragments, and in the absence of precise data on the trajectory of the meteorite the discovery of such fragments 50 years after the fall is a matter of pure chance. Fine meteoritic dust in solid form is scattered and it is difficult to distinguish it from ground formations unless it is composed of pure unoxidized iron. The atomized fused meteoritic material should have frozen into spheroidal particles that could be carried for great distances by the wind to form local accumulations. The gaseous products and their derivatives do not form distinct local accretions and any search for these would be virtually hopeless. The deposits of these various product types need not in principle coincide with one another.
Of all of the enumerated forms of meteoritic matter, the 1958 expedition (Florenskiy et al.) identified only magnetite and silicate spheres, and these in negligible concentrations. Additional specimens were collected by B.I. Vronskiy in 1959 and 1960. All attempts (magnetometry, metallometry, radiometry) to ascertain the presence of cosmic matter in some other form (1958, 1959, 1960) remained entirely without result. At the same time, however, the limited concentration of such particles precluded definite classification as fragments of the Tunguska meteorite, since they are indistinguishable from the ubiquitous meteoric dust.
The question of meteoric (cosmic) dust is important and repeatedly raised in the literature. However, it had not been the subject of an adequately serious study until recently, and quantitative estimates of meteoric dust varied over a very wide range: the estimates of various authors sometimes differed by many orders of magnitude. The properties of this dust are generally studied by astronomical methods, with only the magnetic component being determined from ground deposits.
Thus the problem of studying the material composition of the Tunguska meteorite is inseparably interwoven with the general study of meteoric dust, and part and parcel thereof.
Fig. 9. A concentration installation at the Khushmo River.
Attempts to isolate material from the Tunguska meteorite were carried out in the following manner.
1. Isolation of dust from the surfaces of tall stumps ("telegraph poles") snapped by the 1908 explosion (Yu M. Yemel'yanov). The method called for removal of the trunk surface, grinding it up, and boiling it to a pulp in water. Large wood particles were removed from the resulting mass by filtration, and the minute fractions were collected. This concentrate was dried for subsequent separation in gravity solution. The extremely laborious operation of collecting enough dust that had settled in the cracks of the trunk surfaces failed to produce any large number of specimens of adequate size. The upshot was a single specimen, collected from a surface of approximately one square meter in the vicinity of the Churgim falls, 3 km to the south of the "explosion" epicenter. Preliminary study of some of the material showed a clear preponderance of terrestrial quartz dust in the presence of minute particles possibly associated with wood residues.
The specimen is of interest since it provides a characteristic of the average dust fall in this region over a period of 53 years, including possible residues of the Tunguska meteorite and other meteor particles.
2. Identification of strata in peat-bog and lake deposits. The identification of the1908 peat layer was accomplished in two ways: a) in the north morass and in the vicinity of Khoy Brook, about 20 square meters of a scorched layer containing a large quantity of ash associated with the 1908 fire was identified; the ash, together with the peat residue, was concentrated by the customary method for soil samples; b) successive layers of peat from the west morass in the bend of the Khushmo River (which had not burned) were identified and set afire for subsequent analysis of ash composition.
Silt specimens from Lake Chekoand the lake in the bend of the River in the west morass were collected for subsequent stratigraphic study (P.N. Paley et al.) with a grab dredge and a swamp drill designed by N.I. P'yavchenko.
The various samplings from the bottom of Lake Cheko (P'yavchenko, Kozlovskaya) revealed extensive development of silt up to 7 meters deep, indicating an ancient origin for the lake (tentatively estimated at 5000 to 10,000 years), thus completely contradicting the hypothesis of the formation of the lake as a result of the Tunguska meteorite fall (V. Koshelev, 1960).
3. The search for meteoritic matter in the soils. The observations made by Soil Scientist Yerokhina confirmed the validity of the soil sampling method which we used in 1958; it was possible with this method to remove a layer 2 to 3 cm thick, including the lower section of the litter and the upper layer (1.5 cm) of the subsoil. With this sampling method, the weight taken from an area of one square meter usually ranges up to 10 kg.
Fig. 10. Spheres of cosmic origin found in the area of the Tunguska meteorite fall.
The average weight of a sampling collected by the two-man teams on foot amounted approximately to 20 kg from a sampling area of two square meters. Wherever possible, the samples were taken from unflooded dry level areas without heavy vegetation cover. The collected specimens were sent to the concentration section (Kozlov, Vronskiy, Malinkin, Gorbunova), where the concentration was accomplished on a vibration table (Fig. 9) and the heavy fraction consisting of particles smaller than 0.25 mm was separated; the specimens were subsequently subjected to magnetic separation and then they underwent preliminary inspection through a binocular magnifier (N.I. Zaslavskaya, G.M. Ivanova, N.P Rodionova). The silicate portion was not studied in the field, and the concentrated specimens were brought back to Moscow for further evaluation. We worked on the assumption that the earlier samples (1958), which had been more crudely concentrated, contained both magnetic (magnetite) and silicate spheres (Fig. 10).With the old concentration method, in which a significant portion of the silicate spheres were lost, these spheres were found to be present in proportions of 3:1. It is not out of the question that the samples include polyhedral particles of cosmic origin; however, a method has yet to be developed for their identification. Under field conditions it is precisely the magnetic spheres that most readily lend themselves to concentration and identification. If we assume that the number of these spheres bears some relation to the total number of cosmic particles, they represent the most convenient indices to the presence of meteoritic matter. Based on these spherical particles, preliminary field analysis of their composition becomes possible, as does a purposive search for the scattering ellipse of the meteorite on the basis of even a slight increase in the concentration of meteoritic matter.
All of the previous samples that were collected in the vicinity of the epicenter (as well as the 1961 samples) exhibited an altogether negligible concentration of spheres. Since the average number of such particles had not been established for the soils, they can easily be attributed to the back ground concentration of meteoritic dust of this type. At the same time, the general pattern of the physical process involved in the scattering of these particles appears completely clear to us. We have already mentioned the dissipation of the meteorite train. In addition, the scattering of the meteoritic matter is governed by the explosive disintegration of the meteorite, accompanied by the liberation of a large quantity of thermal energy. This inevitably led to the formation of a powerful ascending current of hot air the analog of the radioactive cloud that rises from a nuclear explosion. Unlike the nuclear explosion of the pure air-burst type, which is virtually free of any dust particles ,the cloud resulting from the Tunguska explosion may have contained a significant quantity of dust and liquid particles, although it was on a considerably lower energy level. From this standpoint, it would more nearly resemble the cloud which results from an atomic explosion at or very near the ground, which draws up a substantial quantity of dust particles.
Without insisting on absolute consistency between the explosions, which involve markedly different levels of energy, we are nevertheless obliged to seek an analogy to the Tunguska fall in nuclear explosions, since they most nearly approximate the fall in terms of force. The base of the mushroom cap of an atomic cloud rises to a height of 8 to 16 km, while the height to which the top of the cloud rises is a function of the force of the explosion and ranges between 20 and 40 km. The cloud produced by the eruption of Mount Bezymyannyy on Kamchatka in 1956 reached a height of 36 km. The dust particles picked up by the thermal currents settle out at a comparatively slow rate and may be scattered far and wide by the wind. Following Stokes' law, the fall velocity of particles ranging in size from 5 to 300 microns is given as v = 0.11d2p m/hr in air, where p is the particle density in grams per cubic centimeter and d is the particle diameter in microns.
Fig. 11. Map showing the distribution of phenomena accompanying the Tunguska meteorite fall. The circles are drawn at 20-km intervals.
1) Approximate boundary of area with trees subject to physiological light scorching- 2) boundaries of felled-timber area and directions of fall- 3) sites of samplings rich (x > 8) in magnetite spheres. The areas of the small circles are proportional to the concentrations of particles; 4) samplings with few magnetite spheres; 5) hypothetical scattering ellipse of the meteorite.
Quartz particles descend from a height of 24,000 meters at the following fall velocities:
Particle diameter, microns . .250 150 75 33
Fall time, hours . . . . . . .1.4 3.9 16 80
Thus, on the basis of particles 150 microns in diameter, which correspond to the largest sizes found earlier, and assuming that these particles had risen to a height of 12km, the time for their descent would be 2 hours.
TABLE 4
Mean Concentration of Magnetite Spheres as a Function of Distance from the Center of the Explosion of the Tunguska Meteorite (Preliminary Data, with the Concentration Given for an Arbitrary Unit of Area)
| Distance from epicenter, km |
Average content | |
|
Number of particles |
Average of samples | |
|
0-10 |
1.5 |
6 |
|
20-30 |
4 |
16 |
|
40 |
8 |
11 |
|
60-80 |
21 |
6 |
TABLE 5
Mean Concentration of Magnetite Spheres for the Southern and Northern Halves of the Investigated Area as a Function of Distance from the Epicenter
| Distance from epicenter, km |
Southern part |
Northern part | ||
|
Number of particles |
Average of samples |
Number of particles |
Average of samples | |
|
20-30 40 60-80 |
5 3 10 |
8 6 2 |
2.5 15 26 |
8 5 4 |
Following accepted practice, assuming the average wind velocity to be 24 km/hr, we find that particles will be carried 50 km from the center of the explosion before settling to earth.
TABLE 6
Mean Concentration of Magnetite Spheres of Cosmic Origin as a Function of Distance from the Epicenter for Averaging over Quadrants (Preliminary Data Concentrations given for an Arbitrary Unit of Area)
|
Distance from epicenter, km |
Northwest |
Southeast |
Northeast |
Southwest | ||||
|
Number of particles |
Average of samples |
Number of particles |
Average of samples |
Number of particles |
Average of samples |
Number of particles |
Average of samples | |
|
20-30 40 60-80 |
1 14 33 |
4 3 3 |
7 3 10 |
5 4 2 |
4 17 5 |
4 2 1 |
2 3 _ |
3 2 _ |
Fig. 12. Concentration of magnetite spheres in the soil as a function of distance from the epicenter: a) Averaging over quadrants from the southeast to the northwest; b) averaging over quadrants from the southwest to the northeast; c) averaging over all samplings, without consideration of direction.
In response to an inquiry from the Committee on Meteorites, the Central Weather Forecasting Institute provided the following description of the day on which the Tunguska meteorite fell (from a letter dated 11 May 1961, Ref. No. K-573): "On the basis of data available to the Institute, on 30 June 1908 the site of the meteorite fall was under the influence of a zero-gradient pressure field, with weak southeasterly winds at 2 to 5 meters per second."
Fig. 13. Recurrence of samples with a specific number of magnetite spheres.
The synoptic situation provides a basis for the contention that there were no strong air currents at heights of 3 to 4 km. The air currents were moving from the southeast and south to the northwest and north. The velocity of the wind ranged approximately between 30 and 40 km/hr. The direction of the air currents remained constant from the site of the meteorite fall to 65-70ш North Latitude. The air currents then deflected eastward, becoming almost westerly, assuming a northwesterly direction over the basin of the Lena River and the Far East."
In view of the rather indeterminate nature of the hypothetical initial data (and in view of completely contradictory data based on a synoptic analysis of the same situation, as obtained through G.F. Plekhanov from Tomsk) it was decided to break the sampling network up into circles with a distance of 20 km separating them and with the sampling areas in checkerboard array (Fig. 11).
Had there been a helicopter at our disposal, we would have been able to complete the entire program of work and to obtain background concentrations as well as the increase in these concentrations at the hypothetical scattering ellipse, of which various versions have been computed. For reasons beyond our control, the entire sampling operation had to be carried out on foot, with the consequence that the results lacked the necessary statistical buttressing and we were able only to discern the probable direction of the scattering ellipse, without any opportunity of describing the contours of the area.
Figure 11 shows the actual sites at which the soil samples were taken. The areas of the stippled circles are proportional to the concentrations of magnetite spheres per arbitrary area unit (Table 4). If we average all of the data over the various distances from the epicenter, we obtain in complete agreement with theory the distribution shown in Table 5, which clearly characterizes the comparative sparsity of particles in the central section and their concentration at the periphery (see also Fig. 12c).
This qualitative relationship fully explains the unsuccessful attempts on the part of previous expeditions to detect a noticeable concentration of meteoritic matter at the epicenter.
A comparison of the northern and southern halves of the investigated area clearly shows the difference in the nature of particle distribution.
The clearest results are given by averaging over quadrants (Fig. 12a, b).
As many as 90 magnetite spheres were found per unit area in the richest specimen - indeed, the only one - taken on the unflooded bank of the Chunya River (10 km above the mouth of the Kimchu River, 80 km north northwest of the epicenter).
It is our opinion that the above relationship is not accidental, although it may be of inadequate statistical certainty. The meteorite's flight path from the southeast in the presence of a south-southeast wind might serve as an explanation, and it would also clarify the slight drift of matter from the meteorite train to the northeastern quadrant (Table 6), with the main mass of the explosion products falling farther to the north and northwest. The scarcity of meteoritic matter in the southwestern quadrant is characteristic.
The indicated distribution pattern for meteoritic matter corresponds to the meteorite trajectory projected by Krinov and to the synoptic conditions of 30 June 1908 as per the data provided by the Central Weather Forecasting Institute.
Zotkin accomplished the preliminary statistical evaluation of these materials. The histogram (Fig. 13) shows the distribution of 36 samples (n) as a function of the number (x) of spheres contained in the sample. If the fractional values of x are combined with the next-lower whole numbers for greater clarity, we obtain the following distribution of n with respect to x from the 28 poor samples (7 spheres or fewer):
| Number of spheres, x | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Number of samples, n | 4 | 13 | 5 | 3 | 0 | 2 | 0 | 1 |
| Frequency, w | 0.14 | 0.46 | 0.18 | 0.11 | 0.00 | 0.07 | 0.00 | 0.04 |
| Probability, p | 0.17 | 0.31 | 0.27 | 0.15 | 0.07 | 0.02 | 0.01 | 0.00 |
| Number of samples expected from formula. ns | 4.9 | 8.5 | 7.4 | 4.3 | 1.9 | 0.7 | 0.2 | 0.05 |
It may be concluded that the poor samples pertain to the background. In this case, the fluctuations in the number of spheres in the sample must be subject to the Poisson distribution:
p(x) = a(x) e-a /x!
where a is the average number (mathematical expectation) of spheres in the sample and p(x) is the probability of x spheres being present in the sample. The Poisson distribution gives the probability that a given number of events will occur in a given interval, provided that they are randomly distributed in the area being studied. The number of meteors noted during the course of a specific interval of time, for example, satisfies the distribution almost exactly. In our case we can assume that the cosmic spheres falling to the earth over an extended period of time will, in general, be uniformly and randomly distributed over its entire territory.
From the preceding conclusion we find that the average a = 1.7 and that the dispersion d2 = 2.6. For this volume of statistical data a = d2 is a satisfactory test. This derivation uses probabilities p calculated from the Poisson distribution, and the theoretical number ns of samples. Agreement with the empirical frequency w and the actual number of samples is also quite good.
The probability of eight or more spheres appearing in the background samples is extremely small, i.e., p (x>8)<10-3. Thus the existing rich samples cannot be explained by fluctuations in the background and point to a certain additional in flow of material to the points at which these samples were taken.
Doctor of Chemical Sciences Paley undertook a field microchemical analysis for nickel in two samples of spheres in order more reliably to classify the subject magnetite spheres as meteoritic matter and differentiate them from industrial dust; the results of this analysis a represented below:
Sampling conditions Ni:Fe, %
Averaged sphere sample from various samplings ...........11.0
Spheres from Vanavara sample, 300 m from airfield .............4.0
These data completely confirm the correctness of classifying the overwhelming majority of the spheres under investigation as meteoritic matter. The significant difference between the averaged sample and the Vanavara sample, clearly exceeding the error of the experiment, can be explained either by accumulation of several generations of cosmic matter (meteoritic dust) in the vicinity of Vanavara, or by some contamination of the sample taken in the immediate vicinity of Vanavara by dust of industrial origin, e.g., from welding apparatus.
The next stage in our work calls for careful inspection of the collected samples, separation and study of the silicate particles, and detailed description of all the various types of extracted meteoritic particles to determine the possible composition of the infusible components of the Tunguska meteorite.
If we assume that the spheres that O.A. Kirova had extracted from the samples taken from this region pertain exclusively to the Tunguska meteorite, and not in any considerable measure to the scattered background of meteoritic dust, we can assign an iron-and-silicate composition to the infusible portion of the meteorite, although for the time being this remains to some extent hypothetical. The total quantity of matter distributed over the investigated area can, in approximate terms, be estimated at several tons.
We believe that the work of the 1961 expedition represents a great forward stride in the search for the meteoritic matter. A distinct concentration of particles of cosmic origin was discovered to the north northwest of the epicenter, fitting well into the theoretical pattern for the scattering of the Tunguska meteorite. However, in view of the fact that the distribution of the over-all meteoritic-dust background has not been thoroughly studied, and because of the inadequate number of samples taken, the composition of the meteorite and the conditions of its fragmentation cannot be regarded as having been determined with complete certainty.
We are now faced with the quite specific problem of looking for particles of a definite type by a specific method and in a specific region. Materials collected in this manner must be investigated as part of a general study of the cosmic dust, which represents an extremely important task for the geochemist.
In conclusion, we feel we must make a number of critical remarks with respect to Zolotov's observations, which provided the basis for a whole series of unfounded statements with regard to the nature of the Tunguska meteorite in the popular literature. Despite the serious criticism of A.V. Zolotov's reports, his works have appeared in the Doklady Akademii Nauk SSSR (see the first issues of Volumes 136 and 140, for the year 1961).
The observations which serve as the basis of these papers are erroneous.
1. From among a mass of trees that had died at various times, Zolotov selected one with a certain arrangement of knags supposedly characterizing the relationship to the ballistic and shockwave near the trajectory of the meteorite. The expedition has at its disposal material indicating surviving lateral stumps oriented in all possible directions, so that a solitary observation without any indication of the time at which the branches were lost is of no significance.
2. In determining the pressure at the front of the shock wave, Zolotov proceeds from the idea that in comparison with tree trunks, an excess air pressure of proportionally reduced intensity would be required to snap off thin branches. However, it is a well-established fact that considerable force is required to snap off the flexible branches of evergreens and that in the case of a windfall the conifer itself generally breaks, retaining virtually all of its branches.
3. The inadmissibility of equating the color temperature and brightness of a bolide with the temperature of the shock wave has been demonstrated in a study by Stanyukovich and Bronshten [10].
4. In his estimate of the luminous energy produced by the explosion, Zolotov identifies the boundary of the forest fire with the boundary of flash burning and bases his calculation on the most distant point of the fire (16 km to the southeast). The spread of the fire to the west, however, was not nearly as great and has absolutely nothing in common with the boundary of flash burning. By no means were the living trees ignited by the initial flash, but rather only such easily fired materials as the forest litter and dead wood. The luminous energy of the explosion was estimated at 60 to 100 calories per square centimeter at a distance of 17 to 18 kilometers from the point of the explosion, which works out to a luminous energy of the order of 600 to 900 calories per square centimeter at the epicenter of destruction, in complete contradiction to observational data, since a rather large number of live trees survived in the vicinity of the epicenter, bearing only traces of physiological cambium scorching, which corresponds to an energy of 6 to 12 calories per square centimeter.
5. Zolotov based his calculations on an unrealistic absorption coefficient of 0.033 km-1 for the atmosphere, which corresponds to a visibility of 120 km; the use of a realistic coefficient of 0.1 km-1, which corresponds to a visibility of 45 km and excellent atmospheric transparency, nullifies all of the calculations, altering the result by several orders of magnitude.
6. Evaluation of the eyewitness accounts suggests no thermal effect nor any of the electrical background phenomena of a great meteor.
In a second paper (Doklady Akademii Nauk SSSR, 1961, Volume 140, No. 1) Zolotov draws the correct conclusion that "...the difference in the conditions of radioactive fallout (contemporary - K.F.) fully explains the variations in the total specific radioactivity of the Tunguska tree and topsoil specimens," as had been pointed out in the report of the 1958 expedition sponsored by the Committee on Meteorites. This Committee has at its disposal data from radiochemical isotope analyses of tree sections by the annual rings, conducted under Prof. V.I. Baranov, and these definitely point to a contemporary nature for the radioactive contamination of the trees, since it does not stop with the tree ring which corresponds to the time of contamination, but rather penetrates the wood to a considerable depth.
The investigation methods used by Zolotov are not up to current standards and thus cannot be used for any evaluation of the phenomena accompanying the fall of the Tunguska meteorite.
III. GENERAL RESULTS
The work of the expedition can be summarized as having virtually completed the collection of materials which will provide descriptions of all the various forms of the physical effects produced by the Tunguska meteorite on the area of the fall. It is, of course, not out of the question that individual craters formed by small pieces of the meteorite will yet be found - or, for that matter, the pieces themselves. However, in view of the projects that have already been carried out, it becomes quite clear that such finds would be only fortuitous.
The maps of the felled-timber area are no longer based on estimates, but are well documented and prepared on the basis of a considerable volume of statistical material. The data collected on the mean direction of the fallen trees, on the characteristics of the forest, on the degree to which the possible flash burning was directional, and on the influence of the terrain relief on the nature of the shock-wave effect at the epicenter must provide the basis for physical characterization of the air "explosion" of the Tunguska meteorite. We have no hesitancy in expressing our conviction that the study of this material will shed light on the over-all physical circumstances of the fall in a quite thorough and definite manner.
The investigation into the distribution of meteoritic dust in the area of the fall permits us, with a high degree of probability, to speak of physically observed fragments from the Tunguska meteorite and the nature of their scattering. However, to transform the probability into full certainty, the distribution of this material must be the subject of study, in conjunction with the general study of cosmic dust and its propagation. At the same time, methods must be worked out to isolate identify cosmic dust - something that will be possible only behind the walls of a major institution such as the Vernadskiy Institute of Geochemistry and Analytical Chemistry, which should coordinate the study of this problem.
I should now like to take advantage of the opportunity to express my heartfelt gratitude to the many individuals who participated in the discussion and implementation of the Tunguska meteorite study program.
List of Participants in the 1961 Combined Tunguska Meteorite Expedition:
|
1. Florenskiy K.P. Chief of expedition |
40. Kulakov Yu. |
REFERENCES
1. K.P. Florenskiy, B.I. Vronskiy, Yu.M. Yemel'yanov, I.T. Zotkin, O.A. Kirova. Preliminary Results from the 1958 Tunguska Meteorite Expedition. Meteoritika, Volume XIX, 1960.
2. G.F. Plekhanov et al. A Report on the Work of the Combined Autonomous Expedition. Tomsk, 1961 (manuscript).
3. Yu.A. L'vov, N.V. Vasil'yev et al. Verification of a Hypothesis. Is the Felled Forest in the Keta River Basin Associated with the Fall of the Tunguska Meteorite. Priroda, No. 7, 1961.
4. B.I. Vronskiy. Priroda, No. 3, 1960.
5. Ye.L. Krinov. The Tunguska Meteorite. USSR Acad. Sci. Press, 1949.
6. O.A. Kirova. A Mineralogical Study of Soil Samples from the Site of the Tunguska Meteorite Fall, Collected by the 1958 Expedition. Meteoritika, Volume XX, 1961.
7. V.I. Nekrasov, Yu.M. Yemel'yanov. Priroda, No. 2, 1962.
8. M.A. Tsikulin. An Approximate Evaluation of the Parameters of the 1908 Tunguska Meteorite Based on the Destruction of the Forest Area. Meteoritika, Vo1ume XX, 1961.
9. K.P. Stanyukovich, V.P. Shalimov. The Motion of Meteoroids through the Atmosphere of the Earth. Meteoritika, Volume XX, 1961.
10. K.P. Stanyukovich and V.A. Bronshten. The Velocity and Energy of the Tunguska Meteorite. Doklady Akademii Nauk SSSR, Volume 140, No. 3, 1961.
11. V.A. Bronshten. On the Motion of the Tunguska Meteorite through the Atmosphere. Meteoritika, Volume XX, 1961.
12 G.I. Pokrovskiy. Possible Mechanical Phenomena in the Motion of Meteoroids. Meteoritika, Volume XX, 1961.
13. I.S. Astapovich. The Great Tunguska Meteorite. Priroda, No. 2, No. 3, 1951.
14. V.G. Fesenkov. The Cometary Nature of the Tunguska Meteorite Astrorlomicheskiy zhurnal, Volume 38, No. 4, 1961.
15. V.I. Vernadskiy. Mirovedeniye, No. 5, 1932.
16. V.I. Vernadskiy. Problemy Arktiki, No. 5, 1941.
17. Resolution of the Ninth Conference on Meteorites with Regard to Studying the Fall of the Tunguska Meteorite. Meteoritika, Volume XX, 1961.
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