ANSWERS FROM TUNGUSKA
by Giuseppe Longo and Romano Serra
(Published on: Meteorite!, 4, p.12-13; 1995)
|Many witnesses of the Tunguska
catastrophe are still alive.
Some of them are two or three hundred years old. They felt the tremendous strength of the blast, their flesh was severely injured, but they remained silent and for more than 80 years kept their secret about the Tunguska event. These mute witnesses are the trees that survived the dreadful explosion of 30 June, 1908.
That day, over the desolate region of the Podkamennaya Tunguska river a cosmic body vaporized in the atmosphere at a height of about 8 km with an energy release which would have been equivalent to more than a thousand Hiroshima bombs. 60 million trees were uprooted, one thousand square kilometers of forest were charred by radiation, innumerable plants and animals were killed over a vast region, but miraculously several trees survived this terrible devastation. After many years of intensive research no impact craters or meteorite fragments have ever been found over an area of 15,000 km 2 , so that the nature and composition of the cosmic body remain controversial.
information about what happened more than 80 years ago, the authors, together with their
colleagues Menotti Galli and Stefano Cecchini, decided to interrogate the only witnesses
still alive, the surviving trees. Our group followed Galli's idea that the fluid resin,
emitted around dead branches during the 1908 vegetation period, could have acted as a
trap, as happens in amber, for aerial particles present at the moment of the event.
One of the wood samples taken in Tunguska, in the form of a transversal stem slice, is photographed in Fig. 1. A branch merged in the stem and surrounded by resin is visible on the tree section in correspondence with the 1909-1924 growth rings. The resin emitted in the preceding years is also present under the surface wood of this sample as shown by the shadow on the older growth rings. Most of our samples were extracted from living trees with a corer. The wounds caused by perforation were immediately treated with an appropriate cream and the trees not only survived the catastrophe but also our intrusion of their privacy (see Fig. 2).
With an electron microscope we examined the resin deposited in the years from 1885 up
to 1930 and we observed more than 7,000 trapped particles whose dimensions were equal to a
few thousandths of a millimeter. The time distribution of these particles shows clear
abundance peaks centred on 1908 for some elements.
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 internal haemorrhage, i. e. of the resin emitted by the wounded tree to protect it against infection. 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 autumnal wood, characterized by narrower cells with thinner walls (see Fig. 3), indicating a reduced lignification in the months following the catastrophe. Defoliation, as a consequence of the explosion damage and heat, is responsible for the minimal width (often less than one or two tenths of a millimeter) of the 1909 growth ring. In 1910-1913, some rings had a very irregular shape, due to a possible compression by the bark 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.
The most spectacular phenomenon observed in all the Tunguska trees
examined is their accelerated growth, usually starting from 1909 but sometimes from some
years later. In the example, Fig. 1, we can see the ring widths after the blast increased
on average by more than four times. This growth has only weakened in recent years, when
the tree reached the respectable age of more than 150 years.
The testimonies collected from surviving trees lead us to the conclusion  that 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
 Longo G., Serra R., Cecchini S. and Galli M., Planetary and Space Science, 42, n.
2, 163-177, 1994.
Giuseppe Longo is Professor of Physics at the University of Bologna
(Italy); he has published more than 60 scientific papers on nuclear physics and, in recent
years, has been involved in research on the Tunguska event.
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