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sample populations: Neither can be affected by terrestrial weathering. Finally, shock effect differences have been confirmed by thermoluminescence studies. This leaves an extraterrestrial cause, probably a time-dependent one, as the source of the difference, but this hypothesis is highly controversial. Monte Carlo (statistical random walk) calculations of asteroid ring éjecta indicate a flux constancy on Earth on a scale of at least 107 yr (26). Hence, meteoroids arriving at Earth from the asteroid ring today and 10 5 -10 6 years ago should be sampling the same sources. However, this need not be the case for Apollo asteroids, which include the only three S-type objects known, i.e., those with spectral properties like those of ordinary chondrites. In 1983, Greenberg and Chapman (27) showed that Apollo asteroids contribute only slightly to the meteor oid flux on Earth over a 200-yr span, such as that represented by non-Antarctic falls. However, calculations prompted by the report of Dennison et al. (22) indicate that on the 10 6 -10 6 year scale of Antarctic meteorites, Apollo asteroidal material should be effectively captured on Earth. Hence, Apollo debris may predominate in the Antarctic population, and asteroid ring debris may be represented by the steady background of contemporary falls. Clearly, many additional tests are needed if this hypothesized meteoroid flux variation can be regarded as proven. Whether it proves Correct or not, the hypothesis has sparked reconsideration of the dynamic processes by which meteorites arrive on Earth. It may turn out that Antarctica is a source of extraterrestrial material from objects not previously sampled, indeed that may no longer exist to be sampled. Conclusion Meteorites are, by their very nature, objects of interdisciplinary interest. Results of one study—say, chemical analysis—often can be applied to other areas, orbital dynamics in the last case considered. Meteorites from Antarctica are used not only to study extraterrestrial processes but also as probes of ice sheet history and dynamics. Early experience gained from meteorite studies provided guidance for proper handling, preservation, and analysis of Apollo lunar samples. Studies of these samples, in turn, led to the development of extremely sensitive techniques that are now being used to analyze meteorites and microgram-sized interplanetary dust particles of probable cometary origin collected by high-altitude aircraft and from sea sediments. The past 30 years
