A Possible Mechanism of Concentration of Meteorites within the Meteorite Ice Field in Antarctica

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991 pieces of meteorites of various kinds have been collected from a bare ice field (now named the "Meteorite Ice Field") near the Yamato Mountains (about 72°S, 36°E) in Antarctica. Among these meteorites, 2 are irons and 1 is a stony-iron, while all the others are chondrites or achondrites, in which 5 pieces of carbonaceous chondrite are included. The differential mass distribution spectrum of these stony meteorites is represented by N(m) ∞m^<-5/3>, which is in agreement with the theoretically expected result for small asteroidal bodies based on the fragmentation model theory. An interpretation that all the meteorites are due to one or several meteorite showers can be rejected because their meteoritic characteristics are broadly dispersed. The most plausible interpretation is such that a large number of meteorites falling over the Antarctic interior during a long period of time have been transported by the ice sheet movements which result in a horizontal convergence and an upwell of ice flow owing to the bedrock topography. Another necessary condition is the exposure of meteorites on the blue ice surface caused by the ablation effect. Semi-quantitative examinations of each process involved seem to justify the above-mentioned hypothesis. Results of the examinations are as follows : (a) The estimated gravity separation effect of meteorite in ice is negligibly small so that all Yamato meteorites should have been transported by the ice sheet flow. (b) The velocity distribution along the streamlines of ice flow estimated on the basis of a new theory of an ice sheet profile gives rise to an estimate of the longest time interval from a meteorite fall to its exposure as (10)^5 years. (c) The bedrock topography comprising the Yamato Mountains and the sub-ice Vernadski Mountains is very likely to cause a horizontal convergence and an upwell of the ice sheet flow in the vicinity of the Yamato Mountains, where triangulation surveys have shown an actual example of the horizontal convergence and the observed upwell velocity ranging from 2 to 8cm/year. (d) Continuous ablation of the ice sheet surface can result in an accumulation of meteorites on the bare ice surface. The annual ablation rate there amounts to 2-7cm/year. It seems that these conditions are favorable for an accumulation of meteorites which have fallen in the accumulation area on the northern slope of the Fuji Divide. A U.S.-Japan cooperative program of Antarctic search for meteorites in West Antarctica, planned on the basis of this hypothetical mechanism, has led to a collection of 11 meteorites from two bare ice areas among theoretically predicted ones.
国立極地研究所の論文

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Nagata Takesi
National Institute of Polar Research

A Possible Mechanism of Concentration of Meteorites within the Meteorite Ice Field in Antarctica

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