DENSITIES OF STRATOSPHERIC MICROMETEORITES

We have measured the densities of roughly 150 5- to 15-mum interplanet ary dust particles harvested in the stratosphere. Care was taken to mi nimize selection bias in the sample population. Masses were determined using an absolute X-ray analysis technique with a transmission electr on microscope, and volumes were found using scanning electron microsco pe imagery. Unmelted chondritic particles have densities ranging betwe en 0.3 and 6.2 g/cm3, averaging 2.0 g/cm3. The low median density indi cates appreciable porosity, suggesting primitive, uncompacted parent b odies for these particles. Porosities greater than about 70% are rare. IDPs with densities above 3.5 g/cm3 usually contain large sulfide gra ins. We find no evidence of bimodality in the unmelted particle densit y distribution. Chondritic spherules (melted particles) have densities near 3.4 g/cm3, consistent with previous results for stony spherules culled from deep-sea sediments. (C) 1994 Academic Press, Inc.