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

Abstract

[1] 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.

Formation of the “ponds” on asteroid (433) Eros by fluidization

“…We concede that in the present experiments our substrates were fairly similar and fairly homogeneous in density. Particle-size sorting occurs rapidly in fluidized beds that are mixtures of differing size and density (Huang et al, 1996;Franzen et al, 2003;Moore et al, 2003).…”

“…In addition, experiments to simulate processes on asteroidal regoliths used a simulant consisting of quartz and iron filings with chondritic Fe/Si ratio. Details of these chondrite simulant experiments were reported by Franzen et al (2003) and Moore et al (2003). In experiments most relevant to the present paper, quartz pebbles (0.5-1 cm in size) were placed on the surface to simulate a coarse-grained fraction (i.e.…”

“…Whereas, Kareev et al (2002) and Haseltine et al (2006) suggested that the trigger to flow could be the release of volatiles from the interior of the asteroid. Franzen et al, (2003) and Moore et al (2003) performed experiments under microgravity conditions and found these fluidization effects were impossible to avoid. Once triggered, they needed very little in the way of a carrier gas.…”