Contribution to the International Conference Tunguska96 (Bologna, 14-17 July 1996)


G. Longo, M. Galli and R. Serra

(Department of Physics of the University of Bologna, Italy)

We have described in previous papers [1-3] our search for microsized particles trapped in the resin of trees that survived the Tunguska catastrophe. The tree growth rings provided information on the age of the resin and therefore on the time when the particulate was trapped. The time distribution of the particles showed clear abundance peaks centered on 1908 for some elements. This made it possible to identify Fe, Ca, Al, Si, Cu, S, Zn, Ti, Ni and other elements as possible constituents of the Tunguska Cosmic Body.

The living trees told us not only about the composition of the exploded body, but also provided information about the shock wave and about the heat caused by the explosion [4-5]. No doubt some phenomena observed in the wood of surviving conifers are direct consequences of mechanical and thermal effects of the 1908 explosion. Many 1908 growth rings of trunks and branches showed traumas, while in the rings grown before 1908 there were visible traces of a kind of resin "internal haemorrhage", i. e. of a possible rupture of preexisting resin ducts when the stress reached locally the breaking value for the cells. On the other hand, also in some growth rings of 1910 and following years, we observed the formation of an anomalous number of resin ducts probably due to a damaged cambium. The 1908 ring itself generally has a normal width, showing that its growth was practically complete on 30 June, 1908. This ring, however, has an anomalously clear late wood, characterized by narrower cells with thinner walls, indicating a reduced lignification in the months following the catastrophe. Defoliation, as a consequence of the explosion damage and heat, is also responsible for the minimal width (often less than 0.1-0.2 mm) of the 1909 growth ring. In 1910-1913, some rings have a very irregular shape, due to a possible compression by the cambium damaged in 1908. Finally, an observation of the tree section as a whole indicates that trees not overthrown by the explosion were left leaning in the leeward side of the shock wave, thus causing an eccentricity in the tree section corresponding to the direction of the shock wave.

Another phenomenon observed in all the Tunguska trees examined is their accelerated growth, usually starting from 1910 but sometimes from some years later. Up to today the cause of the anomalous growth is controversial. The fact that the markedly accelerated growth was observed not only in surviving trees, but also in younger trees germinated after the catastrophe has been interpreted by some authors as a proof of genetic mutations ascribed to a nuclear explosion. However, we have found no trace of a nuclear process by examining the radiocarbon abundance in the 1903-1916 tree rings of one of our samples [2].

Some researchers have found correlations between the anomalous tree growth and the position of the trees. They have explained their findings by hypothesizing a scattered fertilization by a "meteoric dust" that encouraged growth in some places and not in others. We collected tree ring data for 9 spruces, 1 larch and 1 Siberian pine. A comparison of the average tree ring width over about 30 years before 1907 and exactly the same period after 1909 has confirmed the width increase for all the 11 trees examined. From these data no correlation with the tree position has been found. The trees were divided into two groups: 5 trees with an average ring width, before 1907, of about 0.4 mm and a second group having in the same period a ring width of about 1 mm. After 1909 both groups reach approximately the same ring width of about 1.2-1.5 mm with an increase for the first group by a factor 3-4, as against a factor 1.2-1.5 for the second group. Thus the trees that grew more slowly before 1908 have been more advantaged by the explosion, with respect to the others. The reason for accelerated tree growth seems to derive from the improved environmental conditions after the explosion: ash fertilization by charred trees, decreased competition for light, greater availability of minerals due to the increased distance between trees, etc. The more favourable conditions were relatively more fruitful for trees that had been more oppressed before the catastrophe and also favoured younger trees born after the explosion, so that the event had an averaging influence on the final tree dimensions.

[1] M. Galli, G. Longo, R. Serra e S. Cecchini, La spedizione al luogo della catastrofe di Tunguska (20 luglio-1 agosto 1991), Il Nuovo Saggiatore, 9, n. 5-6, pp. 85-94, 1993.

[2] G. Longo, R. Serra, S. Cecchini and M. Galli, "Search for microremnants of the Tunguska Cosmic Body", Planetary and Space Science, 42, n. 2, pp. 163-177 , 1994.

[3] R. Serra, S. Cecchini, M. Galli and G. Longo, "Experimental hints on the fragmentation of the Tunguska Cosmic Body", Planetary and Space Science, 42, n. 9, pp. 777-783 , 1994.

[4] G. Longo and R. Serra, Some answers from Tunguska mute witnesses, Meteorite! 4, pp. 12-13, 1995.

[5] G. Longo, "Zhivyie svideteli Tungusskoj katastrofy", Priroda, 1, pp. 40-47, 1996.