Size and density sorting of metal and silicate grains under microgravity conditions and the origin of chondrites

Abstract

We report here experiments on NASA's KC-135 microgravity facility aimed at investigating metal-silicate fractionation in chondrites. Metal and sand particle mixtures, with sizes and relative proportions approximating chondritic values, were allowed to undergo density and aerodynamic sorting as air was passed through 310, ten-centimeter long 2.54 cm diameter columns under microgravity conditions. Metal and silicate fractionation was found to occur when gases passed through the mixtures and when the particle diameter ratio was large (∼3.0), with sand being enriched near the surface of the beds. This is contrary to experiments performed under terrestrial gravity and the semi-empirical Ergun equation. While the reason for this discrepancy is not well understood, the present results do suggest that separation of metal and silicates will readily occur under microgravity conditions on the surface of asteroids.