DENSITIES OF STRATOSPHERIC MICROMETEORITES

We have measured the densities of roughly 150 5- to 15-mum interplanetary dust particles harvested in the stratosphere. Care was taken to minimize selection bias in the sample population. Masses were determined using an absolute X-ray analysis technique with a transmission electron microscope, and volumes were found using scanning electron microscope imagery. Unmelted chondritic particles have densities ranging between 0.3 and 6.2 g/cm3, averaging 2.0 g/cm3. The low median density indicates appreciable porosity, suggesting primitive, uncompacted parent bodies for these particles. Porosities greater than about 70% are rare. IDPs with densities above 3.5 g/cm3 usually contain large sulfide grains. We find no evidence of bimodality in the unmelted particle density 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.