부산대학교 도서관

Emma Rigby, Melissa Symonds and Derek Ward-Thompson review the evidence for the possibility that a comet may have impacted the Earth in historical times, and discuss the size of the putative comet. A global climatic downturn has previously been observed in tree-ring data associated with the years AD 536–545. We review the evidence for the explanation of this event which involves a comet fragment impacting the Earth and exploding in the upper atmosphere. The explosion would create a plume, such as was seen during the impact of comet Shoemaker-Levy 9 with Jupiter. The resulting debris deposited by the plume on to the top of the atmosphere would increase the opacity and lower the temperature. We calculate the size of the comet required, and find that a relatively small fragment of only about half a kilometre in diameter could be consistent with the data. We conclude that plume formation is a by-product of small comet impacts that must be added to the list of significant global hazards posed by near-Earth objects. [ABSTRACT FROM AUTHOR]