The size distributions of nanoscale Fe-Ni-S droplets in Stardust melted grains from comet 81P/Wild 2


  • Nathan E. SANDERS,

    1. Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
    2. Department of Astronomy, Harvard University, Cambridge, MA 02138, USA
    Search for more papers by this author
  • Michael A. VELBEL

    1. Department of Geological Sciences, 206 Natural Science Building, Michigan State University, East Lansing, MI 48824–1115, USA
    Search for more papers by this author

Corresponding author. E-mail:


Abstract– To constrain the effects of capture modification processes, the size distribution of nanoscale refractory Fe-Ni-S inclusions (“droplets”) was measured in five allocations extracted from throughout the depth of Stardust Track 35. The Fe/S ratio has been previously shown to increase significantly with penetration depth in this track, suggesting increasing capture-related modification along the track. Astronomical image analysis tools were employed to measure the sizes of more than 8000 droplets from TEM images, and completeness simulations were used to correct the distribution for detection bias as a function of radius. The size distribution characteristics are found to be similar within independent regions of individual allocations, demonstrating uniformity within grains. The size distribution of the Fe-Ni-S droplets in each allocation is dominated by a mode near 11 nm, but is coarse-skewed and leptokurtic with a mean of ∼17 nm and a standard deviation of ∼9 nm. The size distribution characteristics do not vary systematically with penetration depth, despite the strong trend in bulk Fe/S ratio. This suggests that the capture modification process is not primarily responsible for producing the morphology of these nanoscale droplets. The Stardust Track 35 droplet size distribution indicates slightly smaller sizes, but otherwise resembles those in carbonaceous chondrite Acfer 094, and chondritic porous interplanetary dust particles that escaped nebular annealing of sulfides. The size distribution of metal-sulfide beads in Stardust’s quenched melted-grain emulsions appears to be inherited from the size distribution of unmelted sulfide mineral grains in comet-dust particles of chondritic character.