CARBONACEOUS MICROMETEORITES FROM ANTARCTICA

Over 100 000 large interplanetary dust particles in the 50-500 mu m si ze range have been recovered in clean conditions from similar to 600 t ons of Antarctic melt ice water as both unmelted and partially melted/ dehydrated micrometeorites and cosmic spherules. Flux measurements in both the Greenland and Antarctica ice sheets indicate that the microme teorites deliver to the Earth's surface similar to 2000x more extrater restrial material than brought by meteorites. Mineralogical and chemic al studies of Antarctic micrometeorites indicate that they are only re lated to the relatively rare CM and CR carbonaceous chondrite groups, being mostly chondritic carbonaceous objects composed of highly unequi librated assemblages of anhydrous and hydrous minerals. However, there are also marked differences between these two families of solar syste m objects, including higher C/O ratios and a very marked depletion of chondrules in micrometeorite matter; hence, they are ''chondrites-with out-chondrules.'' Thus, the parent meteoroids of micrometeorites repre sent a dominant and new population of solar system objects, probably f ormed in the outer solar system and delivered to the inner solar syste m by the most appropriate vehicles, comets. One of the major purposes of this paper is to discuss applications of micrometeorite studies tha t have been previously presented to exobiologists but deal with the sy nthesis of prebiotic molecules on the early Earth, and more recently, with the early history of the solar system.