The conference was well attended by both Russian and western scientists, providing a fruitful exchange of ideas and exposing many western scientists, such as myself, for the first time to the extensive research works of our eastern counterparts. There were three main themes of the meeting: a review of results of the extensive studies of the event itself, theoretical studies of the entry dynamics of the bolide, and finally the hazard posed to humanity by Tunguska-like events. Papers on the first topic, understandably mostly Russian, reviewed the essential features of the event, the pattern of knocked-down trees, eyewitness accounts, and various searches for identifiable traces of extraterrestrial material from the bolide. The classic report of this work in the western literature is a paper by Krinov (1966). With regard to finding traces of the bolide, some success is at last being reported. Longo and his colleagues have identified anomalous elemental concentrations in the tree resins dating from the time of the event, and a curious pattern of accelerated growth (cosmic fertilizer?) of the surviving trees. More probably it is due to reduced competition among the survivors. Anomalous elemental abundances have also been identified in the peat layers dated to the time of the bolide event. The essential results of these investigations are the energy released (around 20 MT), the height above ground of the detonation of the bolide (5-8 km), that very little larger than dust made it to the ground, and that a great deal of material was deposited in the upper atmosphere, resulting in enhanced "white nights" lasting for some weeks. For many years these features have been interpreted as requiring that the Tunguska bolide was a comet rather than a stony asteroidal body. (In fact , the post-impact enhancement is easily explained by dust loading of the stratosphere by either a cometary or asteroidal body.) In the second session, devoted to theoretical modeling of the event, the cometary nature of the Tunguska body was challenged. The essential points are that an icy body of the size needed to produce the energy of the Tunguska event would explode 25 km or more above the ground, whereas a stony body would explode right in the range observed and inferred from the tree fall pattern. (See, for example, Hills and Goda 1993 or Chyba, Thomas and Zahnle, 1993. Both papers argue strongly for a stony composition, a conclusion which has been generally accepted in the West but has not penetrated very deeply into Russia, where the impactor continues to be referred to as a comet). There are of course model uncertainties, so not all participants agreed that these results were definitive. One should keep in mind that we really don't actually know what a comet is. There has been an evolution in the paradigm from "dirty iceball" to "icy mudball" structure, and the truth may be that comets are mostly silicate material, and perhaps even quite hard, with just enough ices to power the coma and tail. The recent entry models also indicate that either an icy or a stony body, upon exploding would completely disintegrate into small particles, most of which would shoot back out of the atmosphere in a plume, like those observed in the comet (or was it?) Shoemaker-Levy impacts on Jupiter. Thus very little residue would be expected on the ground in the impact zone, and the observed "white nights" would be expected, regardless of whether the bolide was cometary or asteroidal. Although the issue of asteroid vs. comet was not resolved, I perceived a slow shifting of attitude among the participants toward a rocky body, whether one calls it a comet or an asteroid. The third topic, on the hazard from Tunguska-like events, was almost a non-event. My own paper, suggesting that such events are nearly insignificant compared to other natural hazards and that it would be too costly to defend against them, went unchallenged since the Russian advocates of "planetary defense" failed to show up. I found the meeting more intellectually stimulating than some others I attended on the same trip, in spite of (or perhaps because of) the fact that the topic was somewhat removed from my regular professional interests. I was also fascinated by the personal story of one of the Russian participants, Wilhelm Fast, an ethnic German from western Russia who was exiled as a child with his family at the time of World War II and spent some years in and out of gulags in Siberia over the next 15 years. I wonder if this is a unique story, or if the exiling of intellectual dissidents from western Russia in the Stalinist era has resulted in a substantial enrichment in the academic environment of Siberia by those who just didn't bother to move back when released.
Chyba, C. F., P. J. Thomas, and K. Zahnle 1993: The 1908 Tunguska explosion: Atmospheric disruption of a stony asteroid. Nature 361: 40-44. Hills, J. G., and M. P. Goda 1993: The fragmentation of small asteroids in the atmosphere. Astron. J. 105, 1114-1144. Krinov, E. L. 1966: Giant Meteorites, Part III, The Tunguska Meteorite, pp. 125-265, Pergamon Press, Oxford. Alan Harris
Jet Propulsion Laboratory |
