Tunguska imprints in Antarctica

Globules collected by Florenskiy's 1961-62 expedition were enriched in iridium, a silvery metal that is abundant in extra-terrestrial bodies but rare on Earth, and con tained other evidence of extra-terrestrial origin. Traces of the Tunguska fireball were not only present in the Tunguska soil; they have also been discovered in an Antarctic ice core. This startling revelation - made in 1983 by Ramachandran Ganapathy, an American expert on extra-terrestrial materials - provided fresh insights into the nature of the Tunguska object.

Ganapathy examined eight globules by a nuclear analytical technique with the aim of finding the answer to three questions: Are the globules truly extraterrestrial? Are all the globules related to each other as would be expected if they originated in the Tunguska explosion? Can these globules be distinguished from the remnants of iron meteorites?

All eight globules contained iridium, nickel, cobalt, gold, chromium, antimony and iron. Their iron content ranged from 76 to 81 per cent, and they were all enriched in iridium, a reliable indicator of extra-terrestrial matter. One of the globules had a whopping 56,900 parts per billion iridium (rocks on the Earth's surface contain only about 0.3 parts per billion iridium; a meteorite may contain as much as 500 parts per billion). The concentration of nickel and cobalt, two elements always found with iridium in extra-terrestrial matter, was also present in cosmic proportions. 'There is no question about it', Ganapathy declared in Nature, 'the spheres are extraterrestrials'.

The identical ratio of iridium and nickel in each globule also proved that they all came from the same extra-terrestrial body. The result also disproved the notion that the globules were from meteors falling continuously on Earth. The high abundance of chromium in some globules, however, indicated that the object was not an iron meteorite.

Ganapathy also reasoned that the eyewitness accounts of the Tunguska explosion, together with his findings, demonstrated that the Tunguska object was vaporised by the explosion in the atmosphere. The question of whether the debris from the event could have reached the stratosphere, and as a consequence been distributed globally, prompted him to search for debris in the Antarctic ice. Old ice and air bubbles trapped in it are precious scientific tools. They can illuminate the past, help answer questions about the present, and assist prediction about the future.

The average rate of accumulation of ice - about 7 centimetres per year at the South Pole - is a reasonably good chronometer for measuring time with depth. For his study, Ganapathy selected a 101-metre ice core drilled in 1974. A sample from a depth of 10.15 to 11.07 metres, which corresponded to 1908, contained four times more iridium than found in earlier years. Ice was similarly enriched in iridium during 1909 and the next few years. Then the iridium concentration dropped back to normal levels. This was the strongest proof that particles from the Tunguska fireball were scattered around the world by atmospheric currents. 'Because this iridium could only be deposited here by means of stratospheric fallout, it should be presented worldwide', he said.

The amount of iridium deposited in Antarctica could be used as a clue to calculate the total amount of atmos pheric fallout from the event. The result: 7 million tonnes of debris. Ganapathy estimated that the object that exploded over Tunguska was a 7-million-tonne, 160-metre-diameter monster. He warned that his estimate did not make a distinction between comets and asteroids. However, he said that the object 'may well have been a stony asteroid'.

A year after Ganapathy's observations, Polish scientist Marek Zbik (now at the University of South Australia) examined 100 black magnetic globules from the Tunguska area. The globules varied both in size (from 7 to 350 micrometres) and shape (spherical, droplet-like, some even broken and damaged). Many of these globules turned out to be of terrestrial origin. The ratio of iridium and nickel in the remaining globules was very close to that observed by Ganapathy, proving them to be of extraterrestrial origin. Zbik's analysis did not prove a definitive link between the globules and Tunguska.

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