A new method is here used to obtain experimental data on the nature and composition
of the Tunguska Cosmic Body. The leading hypothesis of the method is that in the forest
conifers which have survived the Tunguska catastrophe, the fluid resin present at the
moment of the event could have acted as a trap for airborne particles, as happens in
amber. The growth rings of trees can give information on the age of the resin and
therefore on the time when the particulate was trapped in them. With a scanning electron
microscope, a total of 7163 particles were found in resin-samples from Tunguska branches
and from two control trees. The time distributions of the particles in the years 1885-1930
show clear abundance peaks centred on 1908. This makes it possible to conclude that the
presence of Fe, Ca, Al, Si, Au, Cu, S, Zn, Cr, Ba, Ti, Ni, C and O in the particles
observed is related to the Tunguska event and that these elements are probable
constituents of the Tunguska Cosmic Body. This result is compatible with recent
calculations showing that the Tunguska body can be a normal density stony asteroid.
On 30 June 1908 the explosion in the atmosphere of a cosmic body over the basin of the
Podkamennaja Tunguska river (Central Siberia) resulted in 2,150 km2 of forest
fall and a great number of trees and branches burnt in a large part of the explosion area.
Since 1927 a great number of scientific expeditions has worked in Tunguska to collect data
on the effects of the explosion and search for fragments or traces of the material from
the disrupted Tunguska Cosmic Body (TCB). Some reviews summarize the results acquired by
these intensive investigations in successive periods, i. e., 1927-1958 (Krinov 1966),
1958-1969 (Vasiljev 1984), 1970-1980 (Vasiljev 1986), 1980-1985 (Vasiljev 1988). Data on
forest devastation and records of the atmospheric and seismic waves have made it possible
to deduce the main characteristics of the Tunguska explosion, i. e. its exact time, 00h
14m 28s UT, (Ben-Menahem 1975), the coordinates of the point usually
called "epicentre", 60° 53' 09"; N, 101° 53' 40"; E, (Fast 1967),
the energy release and height of the explosion, though the values for the last two
parameters are estimated with great uncertainty. However, no impact craters or meteorite
fragments have ever been found in an area of 15,000 km2 (Vasiljev et al.
1990), so that the nature and composition of the TCB are still controversial.
Apart from less conventional hypotheses involving near critical fissionable material
(Zigel' 1959, Hunt et al. 1960), antimatter meteors (Cowan et al. 1965),
tiny black holes (Jackson and Ryan 1973) or alien spacecrafts (Kazantsev 1946, Baxter and
Atkins 1976), the two most plausible explanations consider the explosion in the atmosphere
at an altitude of 5-10 km of a comet (see, e. g., Whipple 1930, Kresak 1978) or an
asteroid-like meteorite (see, e. g., Kulik 1939, Sekanina 1983).
In the recent past the cometary hypothesis has been favoured on the basis that a low
density object was needed to explain the Tunguska catastrophe (Petrov and Stulov 1975,
Turco et al. 1982). Subsequently, to account for the concentration of energy release of
the explosion, two sub-versions of this hypothesis have been developed, one introducing
chemical reactions (Tsymbal and Shnitke 1986), the other nuclear-fusion reactions
(D'Alessio and Harms 1989).
On the other hand, it has been shown (Grigoryan 1976, Grigoryan 1979, Passey and Melosh
1980, Levin and Bronshten 1986) that the fragmentation of a normal density object can
greatly increase the rate of energy deposition in a small region near the end of the
trajectory, thus appearing as an atmospheric explosion. Detailed calculations which
include the effect of aerodynamic forces that can fracture the object, 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). However, due to the uncertainty of such input parameters as the energy and height
of the explosion or the inclination angle and the encounter velocity of the impactor, the
same calculations do not exclude the possibility that the TCB was a high velocity iron
object, nor rule out a carbonaceous asteroid as an explanation of the event.
The only way to achieve certainty about the nature and composition of the TCB remains the
search for some of its remnants. Many radiocarbon analyses of Tunguska wood samples
(Nesvetajlo and Kovaliukh 1983), chemical analyses of soil or plants, bed-by-bed chemical
analyses of the peat formed by Sphagnum fuscum in 1850-1950 (Vasiljev et al.
1973, Golenetskij et al. 1977a, Golenetskij et al. 1977b, Kolesnikov et
al. 1977), isotopic analyses of many different soil, peat and wood samples (Kolesnikov
et al. 1979), as well as analyses of the spherules from Tunguska soil samples
collected in a radius of several tens of kilometres from the epicentre (Jéhanno et al.
1989, Nazarov et al. 1990), have been carried out. Nevertheless, many conclusions
of this intensive work are still uncertain, so that further investigations are needed.
In the present work a new method is used to obtain experimental data on the nature and
composition of the TCB. Preliminary attempts (Cecchini et al. 1992, Valdré and
Korlevic' 1993) to use this method have shown its validity, although the necessary
corrections (Galli et al. 1993) had to be introduced in order to deduce a reliable
dating for the particles.
The leading hypothesis of the method is that in the forest conifers which have
survived the Tunguska catastrophe, the fluid resin freshly emitted due to some
physiological effect, could have acted as a trap as happens in amber, for airborne
particles present at the moment of the event. The most interesting case is when the resin
is emitted and deposited around small dead branches, naturally decorticated long before
the event and subsequently merged in the growing stem. The resin produced inside the
sapwood is partially emitted year by year to the exterior, in a fluid state, during the
most active vegetation period. The part of it immediately surrounding the branch on the
outer bark of the tree, solidifies and becomes gradually included from outside, by the
growing wood. So, the fresh resin can act like a trap for airborne microparticles and part
of them, embedded in the tree-resin, can eventually be found in the resin layer along the
limit between the growth rings and the withered branch. Then, the elemental composition of
the particles can be determined by the analytical electron microscopy technique.
The growth rings, being rather easily datable year by year, can give information on the
age of the resin and therefore on the time when the particulate was trapped in it. To date
the resin, the width of the outside bark and the width of the resin layer on the bark must
be taken into account because they are responsible for a certain shift between the date of
the particle capture and that of the adjacent tree rings, as can be seen in Fig. 1.
Fig. 1. Sketch of the inclusion
mechanism of airborne particles in the resin emitted around a dead branch of a living
The sketch in Fig. 1 shows a vertical section of a stem slice containing
a withered branch. In the upper part of the figure, the slice is represented in the 1908
state with its last growth ring adjacent to the bark. In the vegetative period of that
year, the fresh resin surrounding the dead branch on the outer part of the bark could trap
airborne particles. In the lower part of the figure the same section is shown some years
later. The solidified 1908-resin with the trapped particles has been included inside the
growing tree and now has become adjacent to the growth ring of 1914 (or to a couple of
rings depending on the relative thicknesses of rings and resin layers). The shift between
the age of the resin and that of the tree-rings varies for different trees and should be
The correction for the bark and resin widths was overlooked in the preliminary attempts to
search for microremnants of the TCB in the tree resin (Cecchini et al. 1992, Valdré
and Korlevic 1993) therefore the particles listed in these preliminary works, having been
trapped in the resin some years before 1908, cannot be related to the Tunguska event.
refinements of the method, suggested by M. Galli (Galli et al. 1993), have
permitted the present systematic analysis and the correct dating of the particles embedded
in the tree-resin before, during and after the Tunguska event.
Previous chemical analyses of materials from Tunguska have generally been carried out by
searching for traces of elements from the TCB in soil, plants and rocks (Kovalevskij et
al. 1963, Iljina et al. 1971, Golenetskij et al. 1977a), in beds of peat
(Golenetskij et al. 1977a, Golenetskij et al. 1977b), in growth rings of
trees, etc... These analyses - in which infinitesimal mass concentrations of some
elements, possibly originated from the products of the explosion of the TCB, were sought
in the whole mass of the materials considered - have given uncertain results. Attempts to
consider spheroidal particles found in the soil as relic candidates for the TCB came up
against the difficulty of reliably dating the spherules. A careful comparison of
their composition with that of spherules of known origin has led to the conclusion that
about 95 % of the spherules examined have a terrestrial or micrometeoritic origin, and the
difficulty of dating the remaining part left uncertain their provenance from the TCB (Jéhanno
et al. 1989, Nazarov et al. 1990). The great advantages of the new method
here used are that it gives the possibility to bring out and analyse particles which could
be pure microremnants of the TCB, and to reliably date them. The same
advantages should reveal themselves also when the method is applied to investigate the
isotopic composition of the elements found.
The refined method can in principle be applied to get information on environmental
conditions in the past, especially in connection with events of microsized particulate
production (aerosols from human, volcanic and seismic activity, forest fires, etc...). The
accumulation in the resin can be thought to be similar to the deposition and accumulation
of volcanic aerosols and particulate in polar ice sheets. So, for example, in the case of
volcanic eruptions, large quantities of aerosols and ashes are injected into the
atmosphere and are generally deposited within 1 or 2 years so that any
"clean-air" location, even at a great distance from the site of the event, must
be expected to receive significant amounts of fall-out, which can be subtracted from the
background and analysed in a way similar to the present one.
To collect the necessary wood samples from living trees that have survived the
catastrophe, the authors of the present work participated, in the summer of 1991, in a
scientific expedition (Cecchini et al. 1992, Galli et al. 1993) led 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 from the Astronomical
Observatory of Tomsk.
Among the few conifers per hectare that have survived the Tunguska catastrophe in the
vicinity of the epicentre, the most frequent are Siberian larches (Larix sibirica)
and common pine (Pinus sylvestris) followed by Siberian spruce (Picea obovata)
and Siberian pine (Pinus cembra). The most suitable tree for our purpose turned out
to be the Siberian spruce, with abundant resin and very regular yearly lignification
cycles (i. e. without double or missing annual rings), so that the latewood tree ring
limits could easily be dated. Therefore, the samples have been taken from trees of this
species plus, for comparison, some samples from a Siberian pine.
The samples collected have the form of thin transversal stem slices, sawn at different
heights, or of cylindrical cores extracted from living trees and containing the embedded
withered branches surrounded by resin.
The samples of interest contained withered branches, dead at least some years before
the catastrophe of 1908. On such branches the living tissues have deposited resin both
from years before and after the catastrophe. The traces of the withered branches could be
discovered by looking in the bark of the stem for spots of shiny fresh resin, the oldest
dead branches being more abundant in the lower part of the stem because of the natural
pruning of the trees in a normally thick forest. The greater part of the samples was
extracted directly from living trees with a 22 mm diameter corer. In order to verify the
adequate age of the trees, preliminary cores were extracted with a 5 mm Pressler probe.
The wounds due to the perforations were immediately treated with an appropriate cream.
The wood samples collected in Tunguska were subsequently examined in Bologna. In order to
bring out the branches, the branch-stem interstices, containing the resin layers, were
opened. It was then possible to proceed to electron microscopy inspection of the samples
in two different ways: a) through scanning of the surface of the resin that was adjacent
to the dead branches; b) by direct observation of the particles extracted after dissolving
the resin inside filtering capsules placed in a Soxhlet extractor.
The overall size, morphology and chemical composition of the particles have been
determined using a scanning electron microscope equipped with an energy dispersive X-ray
spectrometer (in the present work a Philips SEM 515, with EDAX PV9100). A computerised
electron detector has been used to determine the concentration of components with atomic
numbers greater than 10. The samples extracted from the surface of the resin have been
placed on standard SEM stubs and coated with thin films of carbon by evaporation in
The resin deposited in the years ¸ 1885-1930 on 14 branches of 7 trees situated in
different directions within a radius of 8 km from the epicentre of the Tunguska
catastrophe, has been examined (see Table 1). The exact location of the trees listed in
Table 1 is given elsewhere (Serra et al. 1994). For comparison, 6 other branches of
a control tree growing at about 1,100 km W-SW from the Tunguska site and the resin
deposited on the roots of a control tree uprooted by the explosion of 1908 have also been
In the 71 resin samples from 20 branches and 3 roots of the 9 trees listed in Table 1,
7163 particles were found up to 1 June 1993. From this total, 171 pure gold particles,
mainly found in the resin of the nineteenth century, clearly having nothing to do with the
Tunguska event, will be discussed elsewhere. Here the other 5854 particles found in the
resin-samples from Tunguska branches are discussed and compared with the 1138 particles
from the two control trees.
The shift between the age of the resin and that of the tree rings has been determined by
measuring the bark-width and the width of the tree-rings in all the wood-samples
considered, taking into consideration the bark-width and the thickness of resin layers on
living younger trees. This shift has been found equal to 6 ± 3 years. The dating given
hereafter is referred to the resin age, calculated with a 6-year shift with respect
to the adjacent tree-rings. Taking into account the uncertainty in the resin dating, three
periods are here considered: 1902-1914, which corresponds to particles that could be
related to the Tunguska event; 1885-1901 and 1915-1930, which correspond to
"background particles" trapped before and after the Tunguska event.
As mentioned above, a chemical semiquantitative analysis of the composition of the
particles has been performed by using a windowed energy dispersive X-ray spectrometer.
Great care has been devoted to avoid contamination of the samples during their preparation
and analysis. The series from the same branch of the resin samples corresponding to the
three periods considered were carbon-coated contemporarily. To exclude artifacts, for six
branches, two or three different series of resin-samples for each branch have been
prepared at different times after a general check of the experimental set-up.
The analysis has revealed the presence of 28 elements in the particles from the Tunguska
tree-resin. In Table 2, the elements present with more than 1 % in at least 1 particle are
listed, taking into account that the equipment used cannot reliably detect quantities <
1 % or elements with atomic number Z < 10. These elements are
divided into two groups. A first group of elements, which is present in all three periods
considered is here indicated as "low-Z" elements. It corresponds to elements
with atomic number Z < 20, plus iron. A second group of
elements, mainly present in the period 1902-1914, is here called "high-Z" and
contains elements with Z >= 22 with the exclusion of iron.
The 5854 particles found in the resin samples from Tunguska branches are split in Table 3
into "low-Z" particles, i. e. particles containing only low-Z elements,
and "high-Z" particles, which contain at least 1 % of 1 high-Z element (the
high-Z particles usually contain a certain percentage of low-Z elements). As can be seen
from the data in Table 3, one third (about 130 particles/cm2) of the
"low-Z" particles could be related to the Tunguska event, but it is difficult to
distinguish them from the two thirds (about 250 particles/cm2) of
"background" particles. Therefore, the analysis is hereafter focused on the
high-Z particles, whose density is increased by more than a factor 10 in the period
Fig. 2. Partition of the 5854
particles found in the resin samples from Tunguska branches for the three periods
The bar graph in Fig. 2 summarises the total number of high-Z and low-Z
particles found in the three periods considered. Due to the fact that for some branches
the resin was available only for one or two of the three periods of interest, the total
number of particles observed is different in the three periods. That is why in the
following graphs the results are presented in per mil calculated with respect to the 1181,
3356 and 1317 total particles found in the corresponding period.
As can be seen from Fig. 3, a clear abundance peak is found in correspondence with the
period of the Tunguska event. Taking into account the ± 3 years uncertainty in the
dating, the particles under the whole peak can be considered related to the
Tunguska event, after subtraction of a very small "background". Similar peaks
have separately been obtained for the different trees examined (Serra et al. 1994).
Fig. 3. Time distribution (per mil)
of the 518 high-Z particles found in the resin samples from Tunguska branches.
The graphs of Fig. 4 show the time distribution of particles containing
a single high-Z element, such as copper, chromium, barium and gold (the last only when
mixed with other elements). Similar peaks appear also for the other high-Z elements for
which sufficient statistics are available, i. e. zinc, titanium and nickel.
Fig. 4. Time distribution (per mil)
of particles containing Cu, Cr, Ba and mixed-Au.
As mentioned, 471 out of the 518 high-Z particles contain a certain
percentage of low-Z elements. For these mixed particles, considered either in their whole
or separately for calcium, iron, silicon, alluminium, sulphur, potassium and chlorine,
mixed with high-Z elements, clear peaks centred at 1908 are obtained, as shown in Fig. 5
for the first 4 cases.
The high-Z particles found in the 1902-1914 resin from Tunguska branches, appear to have
experienced heating and melting. Many of them are spherules or rounded particles like
those shown in the micrographs of Fig. 6.
Fig. 5. Time distribution (per mil)
of particles containing a mixture of low-Z and high-Z elements and of particles containing
Ca, Fe, Si mixed with high-Z elements.
Fig. 6. Micrographs of four high-Z
particles found in the Tunguska branch-resin:
a) a spheroidal particle containing P (34%), Ca (30%), K (18%), Fe
(6%), Zn (4%), Ba (4%) and Si (3%) (tree n° 7, year 1904),
b) a rounded particle containing Fe (77%), W (7%), Al (4%), Cl
(3%), K (2%), S (2%), Zn (2%), Cu (1%) and Mn (1%) (tree n° 1, year 1910),
c) a particle containing Ti (75%), Ca (14%), Si (9%) and K (2%)
(tree n° 7, year 1904),
d) a particle containing Al (28%), Zn (23%), Ca (17%), Si (13%),
Fe (10%), S (3%), K (2%), Cl (2%) and Ti (1%) (tree n° 6, year 1907).
Fig. 7. Micrographs of two low-Z
particles found in the Tunguska branch-resin:
upper) a particle containing Ca (65%) and Si (35%) (tree n° 1,
lower) a particle containing Si (77%), Ca (9%), Fe (7%), Al (4%)
and K (2%) (tree n° 6, year 1906).
Among them sharp-edged or flocculent particles are generally absent,
unlike what happens for part of the low-Z particles trapped in the resin of the
three periods considered (see Fig. 7). The dimensions of the particles can be estimated
from the scale bars on the micrographs.
The spheroidal shape is a surely distinctive feature of particles which underwent a
thermal effect. Out of the 5854 particles from the Tunguska branches, 88 are spheroidal
(77 spheroids are high-Z particles and 11 are pure low-Z particles). The time distribution
for the 88 spheroidal particles as well as that for 54 spheroids containing copper, show
the usual peaks in correspondence with the Tunguska event, as shown in Fig. 8. Similar
graphs are obtained also for the 31 and 30 spheroids containing aluminium and calcium,
respectively. It should be noted that the products of a high temperature fusion followed
by a rapid cooling are very different one from another, as was experimentally checked by
melting meteorite fragments in an electric arc. For example, some spherules obtained in
these conditions contain a very high percentage of nickel, while this element is
completely absent in others (Florenskij et al. 1968).
On the basis of the chemical semiquantitative analysis performed, the "average
composition" ( % ) of the 463 high-Z particles of the period 1902-1914 has been
calculated and is shown by black bars in Fig. 9. The quotes mean that the "average
composition" should be considered only as a qualitative indication. Indeed, as
previously pointed out, the equipment used cannot reliably detect elements with Z < 10
or quantities < 1 %. Furthermore, the measured composition
depends on the place where the electron beam hits the particle and on the angle between
the electron beam and the surface of the particle. In averaging the composition, the
masses of the particles have not been taken into account. Finally, this "average
composition" refers to high-Z particles only. Taking into account that probably about
one third (see Table 3) of the "pure" low-Z particles has the same origin as the
high-Z particles, the percentage of low-Z elements should be increased. It is worthwhile
to note, however, that this qualitative composition clearly differs from the composition
of the 79 high-Z particles found in the root-resin of control tree n° 2 and of the 29
high-Z particles found in 1902-1930 in control tree n° 10 (see Fig. 9).
Upper: time distribution (per mil) of all spheroidal particles.
Lower: time distribution (per mil) of spheroidal particles
After completion of the present analysis, some measurements have been
performed with a new scanning electron microscope, a Philips PXL30 equipped with an EDAX
PV9900 spectrometer. With the equipment now available, it has been possible to push the
analysis down to elements with Z = 5. Two spectra of newly examined particles and the
corresponding micrographs, are given in Fig. 10. As can be seen, these spectra clearly
reveal the lines of oxygen. To search for the presence of carbon, some resin-samples have
been coated with gold instead of carbon. Thus, it has been possible to ascertain that many
elements listed in Table 2 are present not only in oxides, but also in carbonates. This
can strongly alter the measured ratio between the different components of the particles.
Thus, for example, due to the presence of oxygen, the copper/aluminium ratio in the
spherule of Fig. 10 is increased by a factor of 1.5 when obtained by using the new
equipment with respect to the data obtained in the present work.
Fig. 10. The spectra and the
corresponding micrographs obtained with an EDAX PV9900 spectrometer for two particles
found in the 1908 resin of tree n° 5.
Left: a spheroidal high-Z particle containing oxides of copper
(86.6 %) and aluminium (13.4 %).
Right: a rounded low-Z particle containing oxides of Si (67%), Al
(24%), Na (5%), K (3%) and Fe (1%).
Fig. 11. The measured radiocarbon
abundance in percent of the modern concentration (pMC) as a function of the tree-ring date
for wood samples from tree n. 1. The points connected with a solid line correspond to the
data from Stuiver and Becker 1993.
Taking into account the results summarised in the present Section, one
can conclude that there are 14 elements, listed in Table 4, whose presence is related to
the Tunguska event. The list in Table 4 is given in decreasing order of importance for the
first 12 elements. The decreasing order takes into account the number of particles, their
average composition and the probable relation to the Tunguska event of part of the
"pure" low-Z particles above the background. Oxygen and carbon, whose presence
has been ascertained, but for which no quantitative data are available, have been added to
the list. The elements Cl, K, Na, Pb and Mn, though listed in Table 2, are not included in
Table 4, their relation to the Tunguska event being dubious, due to the low overall
percentage of the first three elements and to peaks not so clear as those of Figs. 3-5 for
Pb and Mn. The rather poor statistics for Mg, P, Sn, Sr, Br, Ag, Bi, Co, Sb, Cd, W (from
11 to 1 particles, in decreasing order) has led to the exclusion of these elements from
As pointed out above, the present method should reveal its advantages also in carrying out
the isotopic analysis of the particles found. Up to now, only the traditional radiocarbon
method, which consists in searching for infinitesimal concentrations of 14C in
the full mass of the tree-ring material, has been applied to study the Tunguska wood. The
radiocarbon analysis of a set of our wood samples from tree n. 1 has been carried out by
R. P. Beukens at the IsoTrace Radiocarbon Laboratory of the Accelerator Mass Spectrometry
Facility at the University of Toronto (Canada). Figure 11 shows the measured values, with
their error bars, of the radiocarbon abundance in percent of the modern (year 1950 AD)
concentration, as a function of the ring date. These results are in general agreement with
those from Stuiver and Becker 1993. Radiocarbon due to neutron production in a possible
cometary impact producing a fusion reaction that might affect the radiocarbon
concentration at ground can be ruled out. The graph suggests an increase of radiocarbon in
the late 1909 and early 1910. A similar increase, which has probably nothing to do with
the Tunguska event, has been found in 1908-1910 tree-rings in previous radiocarbon
analyses of Tunguska wood samples (Nesvetajlo and Kovaliukh 1983). This increase may be
related with the expected solar cycle modulation of the cosmic radiation (Attolini et
al. 1989), though the large experimental error in the 1910 point leaves doubts on such
The sudden increase in the resin samples from Tunguska branches of particles
containing the elements listed in Table 4 cannot be related to volcanic eruptions or
tectonic activity. The dates and places of such events do not correspond to the present
data and the morphology of the particles observed is different.
None of the known volcanic eruptions (Simkin et al. 1981) can explain the peaks
shown in Figs. 3-5 and 8. In the period of interest the eruptions that have significantly
affected light transmission in the northern hemisphere with a certain observed dust veil
are those of the volcanoes Soufriére in West Indies (1902), Santa Maria in Guatemala
(1902), Ksudach in Kamchatka (1907) and Katmai in Alaska (1912). The last is apparently
the only one after 1908 able to cause a noticeable decrease in atmospheric light
transmission (Hoyt and Frölich 1983, Lamb 1988). On the other hand, the study of the
content of Antarctic snow (Boutron 1980) has shown that the strong peaks observed in the
concentration of Zn, Pb, Cu and Cd, appear to parallel rather well the variations of
global volcanic activity. In the present work, no peaks of particles containing these
elements have been observed in the years 1902-1904 or 1912-1914, which might correspond to
three of the four volcanic eruptions mentioned. An increased concentration of the same
metals in snow and plants has been observed after earthquakes and it has been shown that
one of the most important sources of metals in the atmosphere are aerosols which are
emitted during degassing in active tectonic zones (Alekseev and Alekseeva 1992). To our
knowledge there is no record of a particular seismic activity that can explain the peaks
centred at 1908, observed in the present work. Therefore, the volcanic and tectonic
activity can account for the presence in the branch-resin of some "background"
Another source of particles in the atmosphere is the heavy metal release from plants
(Beauford et al. 1975, Beauford et al. 1977), which can obviously be
excluded as an explanation of the time-distribution data found for the particles from
Nor can the extraterrestrial dust particles (see, e. g., Sandford 1987, Bradley et al.
1988) explain the time distribution of the particles observed, though they are likely to
be responsible for part of the "background" of about 250 low-Z particles/cm2
and 5 high-Z particles/cm2 found in the Tunguska branch-resin in the three
periods considered (see Table 3). Anthropogenic sources of aerosols cannot explain the
Tunguska data, though they may be responsible for the hint of an increase of high-Z
particles observed in the control tree n° 10 (40 km from Tomsk) in the second and third
decades of the present century. These conclusions are confirmed by the results of the
investigation of heavy metal deposition in the peat of the basins of Podkamennaja and
Nizhnjaja Tunguska rivers (Bojarkina et al. 1986). A bed-by-bed neutron activation
analysis of the peat from different places located more than 80 km from the smallest
inhabited centres and more than 700 km from the nearest industrial cities (Krasnojarsk,
Irkutsk and Norilsk) was performed. The yearly deposition per square meter of heavy metals
was found more or less constant for the period 1765-1972 and equal to 4000 ± 1000 µg for
iron, 5 ± 1 µg for scandium, 3 ± 1 µg for cobalt and 1.2 ± 0.2 µg for gold. These
values, therefore, correspond to the deposition in the Tunguska region from all natural
sources and can be considered as an upper limit for extraterrestrial dust deposition.
These measurements do not provide information about the Tunguska event due to the fact
that the yearly deposition was deduced from the content of finite thickness bed peats, one
of which corresponds to the whole period from 1891 up to 1910. Control measurements
performed by the same authors near an industrial centre showed an increase in the
deposition from 1903 to 1978 by factors of about 19 (Fe), 7 (Co and Sc) and 5 (Au).
The foregoing considerations lead to the conclusion that the increase in 1908 of particles
containing the elements listed in Table 4 is directly correlated to the Tunguska event.
The data obtained from the Tomsk control tree n° 10, not showing such an increase,
favour this interpretation. It is unlikely that the particles observed correspond to the
part of the material of the TCB that was lifted into the higher atmosphere and
precipitated worldwide in the following months. This material was dispersed over a very
large area and there is no reason for a particular intensity of the precipitations in the
vicinity of the epicentre.
A second possible relation with the Tunguska event is that the particles observed were
transported from the ground to the tree-resin by the blast wave from the explosion. A
strong argument against this hypothesis is the morphology of the particles observed in the
peaks centred at 1908. As shown in Fig. 6, the particles have no sharp edges and show
evidence of drastic heating, unlike what one would expect for particles coming from the
ground. Moreover, it is difficult to see a relation between the list of Table 4 and the
numerous chemical analyses of Tunguska soil, plant and rock samples carried out by
different groups (see, e. g., Kovalevskij et al. 1963, Iljina et al. 1971,
Golenetskij et al. 1977a). Finally, if the particles were transported from the
ground, a similar composition would have been found for the particles trapped by the resin
on the roots of control tree n° 2 and those on the branch-resin, but this does not
happen, as Figure 9 shows. Though this graph should be considered with caution due to the
fact that the roots examined pertain to one tree only, it favours the hypothesis of a
different provenance for the particles embedded in the resin of the roots and those from
branch-resin. Indeed, at the moment of the explosion, the resin on the branches was ready
to receive the particles, while the roots were under the ground. The roots emerged and
began to emit resin only several tens of seconds later. Due to this time delay the
root-resin could not receive particles directly from the exploded body but was able to
trap particles lifted from the ground.
Though a provenance from the ground of part of the trapped particles cannot be excluded,
it seems reasonable to assume that at least the main part of the particles above the
background came directly to the branch resin inside the blast wave generated by the
explosion of the TCB. This conclusion is corroborated by the fact that the particles show
evidence of heating and melting as it should be, taking into account that the shocked
gases, concentrated in front of the impacting body, attained a temperature of
25,000-30,000 K (Chyba et al. 1993). Therefore, the elements listed in Table 4 are
probable constituents of the TCB and the present results can be considered as experimental
evidence in favour of the hypothesis of the disruption of an ordinary stony meteorite.
An isotopic analysis of some elements present in the particles found (whose masses are
generally equal to fractions of a nanogram) would be of great value in order to
distinguish between a cosmic or terrestrial origin of the particles. This isotopic
analysis can also help to definitely answer the question of nuclear processes invoked by
some authors to explain the Tunguska explosion. No trace of a nuclear process has been
found by using the traditional radiocarbon method applied to the full mass of the
tree-ring material (see previous Section) and no convincing evidence in favour of such a
hypothesis has ever been found (see, e. g., Kirichenko and Grechushkina 1963, Emeljanov
1963). It should be noted, however, that the methods used up to now cannot be considered
conclusive since they generally deal with radioactivity measurements carried out more than
half a century after the event or with the search for infinitesimal isotopic anomalies in
the whole mass of tree-rings, sample of peat, etc..., resulting in measures at the limit
of experimental possibilities.
The branch-resin method here used has shown its validity in investigating past events
accompanied by aerosol production. Its application to the study of the Tunguska event has
made it possible to recognise a first list of 14 elements as probable constituents of the
TCB. This list of elements is fully compatible with recent calculations (Chyba et al.
1993, Hills and Goda 1993), showing that the TCB can be a normal density meteorite.
Isotopic analysis data together with an extension of the chemical analysis would be useful
to confirm the present conclusions.
The authors gratefully acknowledge G. V. Andreev, E. M. Kolesnikov, V. D.
Nesvetajlo and N. V. Vasiljev for their help in Tunguska and useful discussions, K.
Korlevic' for sending to M. Galli the first Tunguska wood sample, whose structure
examination excited the idea of the branch-resin method here used, G. Valdré for having
carried out a preliminary search of particles trapped in the resin of the same sample and
R. P. Beukens for the radiocarbon analysis of a set of our Tunguska wood samples. The
authors are indebted, for their assistance in using different electron microscopy
facilities, to G. Tarroni and T. De Zaiacomo from the Bologna Centre of ENEA, to F.
Sandrolini and A. Saccani from the Department of Engineering of the University of Bologna
and to A. Garulli, F. Corticelli and D. Govoni from the LAMEL laboratory of CNR in
Alekseev, V. A. and N. G. Alekseeva, Investigation of metal transfer in the
biosphere during gaseous emission in zones of tectonic activity using methods of
nuclear-physics, Nucl. Geophys. 6, n° 1, 99-110, 1992.
Attolini, M. R., M. Galli, T. Nanni and P. Povinec, A cyclogram analysis of the
Bratislava 14C tree-ring record during the last century, Radiocarbon 31,
n° 3, 839-845, 1989.
Baxter J. and J. Atkins, The Fire Came By, Doubleday, Garden City NY,
Beauford, W., J. Barber and A. R. Barringer, Heavy metal release from plants
into the atmosphere, Nature 256, 35-37, 1975.
Beauford, W., J. Barber and A. R. Barringer, Release of particles containing
metals from vegetation into the atmosphere, Science 196, 571-573, 1977.
Ben-Menahem, A., Source parameters of the Siberian explosion of June 30, 1908,
from analysis and synthesis of seismic signals at four stations, Phys. Earth Planet.
Inter. 11, 1-35, 1975.
Bojarkina, A. P., N. V. Vasiljev, G. G. Gluchov, Ju. A. L'vov, V. I. Rezchikov and
S. I. Shcheludko, K otsenke kosmogonnogo pritoka tjazhelych metallov na poverchnost'
zemli, in Kosmicheskoje veshchestvo i zemlja, 203-206, Nauka, Novosibirsk, 1986.
Boutron, C., Respective influence of global pollution and volcanic eruptions on
the past variations of the trace metals content of Antarctic Snows since 1880's, J.
Geophys. Res. 85, n° C12, 7426-7432, 1980.
Bradley, J. P., S. A. Sandford and R. M. Walker, Interplanetary dust particles,
in Meteorites and the early solar system, 861-895, The University of Arizona Press,
Cecchini, S., M. Galli, G. Longo and R. Serra, Identikit di un'esplosione, Sapere
944, 9-15, 1992.
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.
Cowan, C., C. R. Atluri and W. F. Libby, Possible anti-matter content of the
Tunguska meteor of 1908, Nature, 206, 861-865, 1965.
D'Alessio, S. J. D. and A. A. Harms, The nuclear and aerial dynamics of the
Tunguska event, Planet. Space Sci. 37, n° 3, 329-340, 1989.
Emeljanov, Ju. M., Radiofotograficheskoje issledovanie srezov derevjev iz rajona
padenija Tungusskogo meteorita, in Problema Tungusskogo meteorita, 153-158,
Izdatelstvo Tomskogo Universiteta, Tomsk, 1963.
Fast, V. G., Statisticheskij analiz parametrov Tungusskogo vyvala, in Problema
Tungusskogo meteorita, part 2, 40-61, Izdatelstvo Tomskogo Universiteta, Tomsk, 1967.
Florenskij, K. P., A. V. Ivanov, N. P. Iljin, M. N. Petrikova and L. E. Loseva,
Khimicheskij sostav kosmicheskikh sharikov iz rajona Tungusskoj katastrofy i nekotorye
voprosy differentsjatsii veshchestva kosmicheskikh tel, Geokhimija 10, 1163-1173,
Galli, M., G. Longo, R. Serra and S. Cecchini, La spedizione al luogo della
"catastrofe di Tunguska" del 1908, in Dentro la Fisica, 237-252, Clueb,
Golenetskij, S. P., V. V. Stepanok and E. M. Kolesnikov, Priznaki
kosmokhimicheskoj anomalii v rajone Tungusskoj katastrofy 1908 g., Geokhimija 11,
Golenetskij, S. P., V. V. Stepanok, E. M. Kolesnikov and D. A. Murashov, K
voprosu o khimicheskom sostave i prirode Tungusskogo kosmicheskogo tela, Astronomicheskij
Vestnik 11, n. 3, 126-136, 1977b.
Grigoryan, S. S., K voprosu o prirode Tungusskogo meteorita, Doklady Akad.
Nauk SSSR 231, n° 1, 57-60, 1976.
Grigoryan, S. S., Motion and destruction of meteorites in planetary atmospheres,
Cosmic. Res. 17, 724-740, 1979.
Hills, J. G. and M. P. Goda, The fragmentation of small asteroids in the
atmosphere, Astron. J. 105, 1114-1144, 1993.
Hoyt, D.V. and C. Frölich, Atmospheric transmission at Davos, Switzerland
1909-1979, Climatic Change 5, 61-71, 1983.
Hunt, J. N., R. Palmer and W. Penny, Atmospheric waves caused by large
explosions, Philos. Trans. Roy. Soc. Lond., A252, 275-315, 1960.
Iljina, L. P., L. M. Slivina, D. V. Demin, V. K. Zhuravlev, L. P. Potekhina, M. N.
Levchenko, B. I. Vronskij, O. A. Egorshin and G. M. Ivanova, Rezultaty spektral'nogo
analiza prob pochvy iz rajona padenija Tungusskogo meteorita, in Sovremennoje
sostojanie problemy Tungusskogo meteorita, 25-27, Izdatelstvo Tomskogo Universiteta,
Jackson A. A. IV and M. P. Ryan, Was the Tungus event due to a black hole? Nature
245, 88-89, 1973.
Jéhanno, C., D. Boclet, J. Danon, E. Robin and R. Rocchia, Etudes analytique de
sphérules provenant du site de l'explosion de la Toungouska, C. R. Acad. Sci. Paris
308, Serie II, 1589-1595, 1989.
Kazantsev, A. P., Vzryv, Vokrug sveta 1, 39-46, 1946.
Kirichenko, L. V. and M. P. Grechushkina, O radioaktivnosti pochvy i rastenij v
rajone padenija Tungusskogo meteorita, in Problema Tungusskogo meteorita, 139-152,
Izdatelstvo Tomskogo Universiteta, Tomsk, 1963.
Kolesnikov, E. M., A. L. Ljul and G. M. Ivanova, Priznaki kosmohimicheskoj
anomalii v rajone Tungusskoj katastrofy 1908 g., II, Astronomicheskij Vestnik 11,
n. 4, 209-218, 1977.
Kolesnikov, E. M. and G. I. Shestakov, Izotopnyj sostav svintsa iz torfov rajona
Tungusskogo vzryva 1908 g., Geokhimia 8, 1202-1211, 1979.
Kovalevskij, A. L., I. V. Reznikov, N. G. Snopov, A. B. Osharov and V. K. Zhuravlev,
Nekotoryje dannyje o raspredelenii khimicheskikh elementov v pochvakh i rastenijakh v
rajone padenija Tungusskogo meteorita, in Problema Tungusskogo meteorita, 125-133,
Izdatelstvo Tomskogo Universiteta, Tomsk, 1963.
Kresák, L.', The Tunguska object: a fragment of comet Encke? Bull. Astron.
Inst, Czechosl. 29, 129-134, 1978.
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.
Lamb, H. H., Weather, climate and human affairs, Routledge, London, 1988.
Levin, B. Yu. and V. A. Bronshten, The Tunguska event and the meteors with
terminal flares, Meteoritics 21, n° 2, 199-215, 1986.
Nazarov, M. A., M. I. Korina, L. D. Barsukova, E. M. Kolesnikov, I. V. Suponev and
G. M. Kolesov, Veshchestvennye sledy Tungusskogo bolida, Geochimia 5, 627-639,
Nesvetajlo, V. D. and N. N. Kovaliukh, Dinamika kontsentratsii radiougleroda v
godichnykh kol'tsakh derevjev iz tsentra Tungusskoj katastrofy, in Meteoritnyje i
meteornyje issledovanija, 141-151, Nauka, Novosibirsk, 1983.
Passey, Q. R. and H. J. Melosh, Effects of atmospheric breakup on crater field
formation, Icarus 42, 211-233, 1980.
Petrov, G. I. and V. P. Stulov, Motion of large bodies in the atmosphere of
planets, Cosmic. Res. 13, 525-531, 1975.
Sandford, S. A., The collection and analysis of extraterrestrial dust particles,
Fund. Cosmic Phys. 12, 1-73, 1987.
Sekanina, Z., The Tunguska event: no cometary signature in evidence, Astronomical
Journal 88, n° 9, 1382-1414, 1983.
Serra, R., S. Cecchini, M. Galli and G. Longo, Experimental hints on the
fragmentation of the Tunguska Cosmic Body, Planet. Space Sci., to be published,
Simkin, T., L. Siebert, L. McClelland, D. Bridge, C. Newhall and J. H. Latter, Volcanoes
of the world, Houtchinson Ross Publishing Company, Stroudsburg, Pennsylvania, 1981.
Stuiver, M. and B. Becker, High-precision decadal calibration of the radiocarbon
time scale, AD 1950-6000 BC, Radiocarbon 35, n°1, 35-65, 1993.
Tsymbal M. N. and V. E. Shnitke, Gazovozdushnaja modelj vzryva Tungusskoj
komety, in Kosmicheskoje veshchestvo i zemlja, 98-117, Nauka, Novosibirsk, 1986.
Turco, R. P., O. B. Toon, C. Park, R. C. Whitten, J. B. Pollack and P. Noerdlinger,
An analysis of the physical, chemical, optical and historical impacts of the 1908 Tunguska
meteor fall, Icarus 50, 1-52, 1982.
Valdré, G. and K. Korlevic, New method for the study of airborne particles
based on tree-resin trapping and analytical electron microscopy, Ultramicroscopy
49, 382-386, 1993.
Vasiljev, N. V, Ju. A. Lvov, B. I. Vronskij, Ju. A. Grishin, G. M. Ivanova, T. A.
Menjavtseva, S. N. Grjaznova and P. P. Vaulin, Poiski melkodispersionnogo veshchestva
v torfah rajona padenija Tungusskogo meteorita, in Meteoritika 32, 141-146, 1973.
Vasiljev, N. V., Istorija izuchenija problemy Tungusskogo meteorita v
poslevojennyje gody (1958-1969), in Meteoritnyje issledovanija v Sibiri, 3-22,
Nauka, Novosibirsk, 1984.
Vasiljev, N. V., Istorija izuchenija problemy Tungusskogo meteorita v
poslevojennyje gody (1970-1980), in Kosmicheskoje veshchestvo i zemlja, 3-34,
Nauka, Novosibirsk, 1986.
Vasiljev, N. V., Istorija izuchenija problemy Tungusskogo meteorita v
poslevojennyje gody (1980-1985), in Aktual'nyje voprosy meteoritiki v Sibiri, 3-31,
Nauka, Novosibirsk, 1988.
Vasiljev, N. V., G. F. Plekhanov and G. V. Andreev, O mezhdunarodnoj programme
issledovanija Tungusskoj katastrofy 1908 goda, in Neperiodicheskije
bystroprotekajushije javlenija v okruzhajushej srede, 201-207, Tomsk, 1990.
Whipple F. J. W., The great Siberian meteor and the waves, seismic and aerial,
which it produced, Quart. J. Roy. Meteorol. Soc. 56, 287-304, 1930.
Zigel', F. Ju, K voprosu o prirode Tungusskogo tela, in Meteoritnyje i
meteornyje issledovanija, 151-161, Nauka, Novosibirsk, 1983.
Trees and resin samples examined.
A) Resin samples from tunguska branches
Siberian spruce (Picea obovata)
Siberian spruce (Picea obovata)
Siberian spruce (Picea obovata)
Siberian pine (Pinus cembra)
Siberian spruce (Picea obovata)
Siberian spruce (Picea obovata)
Siberian spruce (Picea obovata)
Resin samples from control trees
||Tomsk Siberian spruce (Picea obovata)
||Roots from Siberian larch (Larix sibirica)
The 28 elements present in the particles from Tunguska tree-resin (more than 1 % in at
least 1 particle). The equipment used cannot reliably detect elements with Z < 10 or quantities < 1 %.
Low-Z elements Na Mg Al Si P S Cl K Ca Fe
High-Z elements Ti Cr Mn Co Ni Cu Zn Br Sr Ag
Cd Sn Sb Ba W Au Pb Bi
Particles in the resin samples from Tunguska branches. Average dimensions of the
particles: 4.5 µm x 3.1 µm (0.5 µm x 0.5 µm < 66 % <= 3
µm x 3 µm; 3 µm x 3 µm < 29 % <= 10 µm x 10 µm).
Elements related to the Tunguska event