Scientists' attempts to track marks and traces of Tunguska began in 1957 when the Russian mineralogist A.A. Yavnel analysed soil samples collected by Kulik in 1929 from the Tunguska site, but these samples were later proved to be of terrestrial origin. Later, mineralogist
O.A. Kirova, who was a member of Kirill P. Florenskiy's 1958 expedition, recovered both magnetite globules and various forms of silicate globules from samples collected from the region of the fall. The magnetite globules were either shiny or dull and their sizes varied from 5 to 450 micrometres. The silicate globules varied in appearance from opaque to completely transparent and their sizes varied from 20 to 350 micrometres. The opaque silicate globules also contained large numbers of gas bubbles and traces of iron oxide. Both types of globule are characteristic of the particles produced by the destruction of a meteorite in the atmosphere.
Florenskiy's next expedition in 1961-62 concentrated on the study of the distribution of meteorite material in the soil. This was the first expedition to use a helicopter to deploy individual groups and transport heavy equipment. Thousands of tonnes of soil samples were collected at regular distances of 10, 20, 40, 60 and 80 kilometres from the epicentre. The analysis of these samples showed brilliant magnetic (magnetite) and glassy (silicate) globules, less than 0.1 millimetre in size. When a map of the distribution of these globules was drawn up, it showed that the globules occurred over a fairly well-defined ellipse, with high concentrations between 60 and 80 kilometres to the northwest of the epicentre. This pattern of distribution was probably the result of the wind direction of the day, which was from southeast to northwest. The appearance of the globules showed that they were formed in the atmosphere as molten matter condensed. Florenskiy was convinced that most of the globules were the remnants of a comet that exploded in mid-air.
In the early 1970s the Soviet scientist G.I. Petrov contended that as the Tunguska meteorite moved through the atmosphere, it rapidly evaporated and, when a large amount of vapour had amassed in front of the travelling body, it exploded and scattered in the atmosphere. This scattered matter then condensed into microscopic balls that settled on the vast Tunguska forest. Sphagnum bogs, which receive mineral nutrients only from the air, are most likely to assimilate such 'meteorite balls'.
As a result, Soviet scientists analysed the peat layers of the sphagnum bogs, which add a distinct layer of peat annually, and discovered fused silicate globules, up to 0.8 millimetres in diameter. The concentration of globules was substantially higher in the 1908 layer than in layers before and after the catastrophe. These globules were composed of rare-earth and heavy elements - the elements present in extra-terrestrial material. In his last paper published in English before his death in 2001, Academician Nikolai Vasilyev expressed doubts about the origin of the globules, as similar globules could also be formed during peat burning. 'The presence of small quantities of meteorite dust cannot be doubted', he wrote in Planetary and Space Science, 'but the problem of their connection to Tunguska remains open'.
Another attempt to study peat was made in the late 1970s by Emil Sobotovich and his colleagues at the Institute of Geochemistry and Mineral Physics in Kiev. After six years of investigation, they concluded that the Tunguska blast was caused by a 4,000-tonne stony meteorite that exploded before impact, scattering particles over a wide region. The researchers presented convinc ing evidence of their claim: a large number of tiny diamonds strewn over the Tunguska region. The team incinerated peat collected from the region of the fall in high-temperature ovens. In the ashes they found many irregularly shaped and extremely hard black grains. Further laboratory examination showed them to be diamonds.
Diamonds are formed under extraordinarily high pressures in volcanic pipes known as kimberlites, and since no such pipes have been found in the Tunguska area, the Kiev researchers concluded that the Tunguska diamonds were formed far from Earth (see the section 'The escapade of a gas' in Chapter Eight which contradicts this claim). Such diamonds are already known to exist in uralites, a class of meteorites, which presumably have been involved in deep-space impacts. These meteorites may originate ultimately from comets. Other researchers have suggested that the Tunguska object was a chunk of comet Encke, a periodic visitor to our skies. To support their claim that the tiny diamonds arrived in a meteorite, Kiev researchers cited high levels of radioactive carbon-14 in the peat. They said that such high levels of carbon-14 are found in meteorites that have been subjected to prolonged bombardment by cosmic rays in space.
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