Carbon atoms come in eight varieties, known as isotopes. Carbon-12 (6 protons and 6 neutrons in the nucleus) is the most common isotope. High-energy neutrons which continuously bombard Earth convert ordinary carbon-12 into radioactive carbon-14 (6 protons and 8 neutrons). Living things go on absorbing carbon-12 and carbon-14 until the time of their death. In the case of trees, carbon-14 is recorded in annual growth rings, which also give the age of trees.
It has been suggested that if the Tunguska explosion had been caused by a comet, the hydrogen contained within it would have been compressed and heated as the comet passed through the atmosphere. Some of the hydrogen might have fused into helium, triggering a nuclear explosion that would generate high-energy neutrons and consequently carbon-14 in the atmosphere. Many scientists have measured carbon-14 in Tunguska tree rings, corresponding to pre- and post-Tunguska years. Examination of tree rings formed in 1908 shows a rise in carbon-14, but not enough to support the idea of an annihilation caused by a nuclear explosion. The rise in carbon-14 is attributed to the solar cycle, in which sun-spot numbers rise or fall over a typical period of eleven years. It has also been suggested that the burn-up of the Tunguska object in the atmosphere would have produced a temperature of a few million degrees, too low for nuclear reactions but high enough to produce carbon-14.
A groundbreaking experimental approach by a team of Italian scientists from the University of Bologna, headed by Giuseppe Longo, has uncovered new remnants of the Tunguska fireball. One of the team members, Menotti Galli, was on the 1989 Tunguska expedition, the first post-Cold War expedition open to international scientists. Galli, a physicist, is an expert on phenomena associated with cosmic radiation, including carbon-14.
During the expedition, Galli realised that the only witnesses to the 1908 blast still alive were the surviving trees. But their testimony was hidden in the resin formed around broken branches after the blast. Like amber, this resin could have acted as a trap for particles present in the atmosphere, including extra-terrestrial particles from the fireball. The resin would harden and form a protective coating around the branch. Eventually the resin would become enclosed within the growing branch. What Galli needed was to examine trees in the blast area. Their annual growth rings would point him to the 1908 sections, and if the fireball had showered any particles on the forest, they could still be intact in those sections of the trees. Bingo!
To collect the samples for examination, Galli and his colleagues, Longo, a nuclear physicist, and Romano Serra, an astronomer, attended the 1991 expedition. 'The Italians, accustomed to sipping espressos under Bologna's endless porticoes, found themselves slaking their thirst with brown swamp water laced with mosquito larvae', says Richard Stone in Discover, describing the 'ten difficult days' spent by the Italian scientists in Tunguska. With or without espressos, they still managed to collect resin deposited between 1885 and 1930 on fourteen branches of seven Siberian spruce trees abundant in resin. The trees were situated in different directions within a radius of 8 kilometres from the blast's epicentre. For comparison, they also collected resin from six branches of a tree growing at about 1,100 kilometres from the Tunguska site, and the roots of a tree blown down by the blast.
Back in Bologna, the researchers used a scanning electron microscope to examine their samples. In all they recovered 5,854 particles from the Tunguska branches and 1,183 particles from the two control trees. Their examination of these microscopic particles showed anomalously high abundance of iron, calcium, aluminium, silicon, gold, copper, titanium, nickel and other elements. Some of these elements are commonly associated with normal-density stony asteroids. This abundance peaked around 1908. Another interesting observation was that the smooth texture and spherical shape of particles from the Tunguska branches showed evidence of heating and melting. 'The blast wave would not have melted particles in the ground, where the conductivity was low', said Longo. 'That means the melted particles came directly from the cosmic body.'
Vasilyev pointed out in 1998 that the elements discovered by Longo's team in tree resin were similar to those found by Russian scientists in peat layers. This effect is most probably connected to the Tunguska body, he said, but for the final identification of the particles found in resin as the Tunguska matter, some additional corroboration is necessary, considering the fact that a large volcanic eruption in Russia on 28 March 1907 had produced a significant dust veil over the Northern Hemisphere for more than a year. 'There is no direct evidence that these materials have anything to do with the Tunguska body. On the contrary, there is good reason to believe we are dealing with fluctuations of the background fall of space dust.'
In the same year, Vladimir Alekseev of the Troitsk Institute for Innovation and Fusion Research in Moscow also expressed doubts on the methods used by Longo's team because of the presence of background particles which could come from volcanic eruptions. He came up with a different approach for hearing the testimony of trees: examining particles more energetic than those of the background. For his study, Alekseev selected a standing larch tree that had survived the Tunguska catastrophe. The tree, located near the epicentre, had a 10-centimetre vertical split in the stem. The split, according to Alekseev, could have been caused by shock waves which, coming from above, exerted a force on the growing tree. He took a wood sample from the split.
After removing resin from the sample, when Alekseev examined it with a high-powered microscope he noticed numerous solid particles up to 50 micrometres in size in the dense wood of the 1908 growth ring. The particles could be divided into four groups: metallic particles with jagged edges; spherical silicate particles; whitish particles; and black graphite-like particles. They had energies high enough to penetrate into the dense wood, and Alekseev was convinced that they were the remnants of the Tunguska body.
Based on the information obtained from the study of these particles, Alekseev proposed the following scenario for the Tunguska blast: the flight of the body was finished by multiple explosions and, therefore, the solid remnants are small particles. The multiple explosions could be responsible for the gunfire-like sounds repeatedly heard by the eyewitnesses of the event. There was a possibility of thermonuclear reaction on the surface of the body at the final stage of its journey in the atmosphere. Like some cosmic bodies, this body was probably enriched with deuterium (hydrogen-2). This deuterium could start a thermonuclear reaction which would convert deuterium to tritium (hydrogen-3). 'As tritium is involved in biological processes and it is radioactive, it can have genetic effects', Alekseev concluded.
During the second Italian expedition to Tunguska in 1999, the scientists not only continued their search for microparticles preserved in tree resins, they also looked for other remnants in the sediments at the bottom of Lake Ceko. This 500-metre wide and 47-metre-deep lake is 8 kilometres from the centre of the 1908 explosion. The group, which included Longo who had also attended the 1991 expedition, used an inflatable catamaran for the geological survey and for the coring operations. This work had two objectives: to check whether the lake is an impact crater of the 1908 event; and to detect mineralogical, chemical and biological evidence of the nature of the Tunguska fireball. Their experimental study showed that though the lake was formed by the impact of a cosmic body, it was definitely formed before the 1908 catastrophe. The core samples collected from the sediment have not yet conclusively been shown to be linked with the fireball.
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