Comet 81P/Wild 2: the size distribution of finer (sub-10 μm) dust collected by the Stardust spacecraft

Price, M.C.; Kearsley, A.T.; Burchell, M.J.; Horz, F.; Borg, J.; Bridges, J.C.; Cole, M.J.; Floss, C.; Graham, G.; Green, S.F.; Hoppe, P.; Leroux, H.; Marhas, K.K.; Park, N.; Stroud, R.; Stadermann, F.J.; Telisch, N. and Wosniakiewicz, P.J. (2010). Comet 81P/Wild 2: the size distribution of finer (sub-10 μm) dust collected by the Stardust spacecraft. Meteoritics & Planetary Science, 45(9) pp. 1409–1428.



The fluence of dust particles <10 μm in diameter was recorded by impacts on aluminium foil of the NASA Stardust spacecraft during a close fly-by of comet 81P/Wild 2 in 2004. Initial interpretation of craters for impactor particle dimensions and mass was based upon laboratory experimental simulations using >10 μm diameter projectiles and the resulting linear relationship of projectile to crater diameter was extrapolated to smaller sizes. We now describe a new experimental calibration programme firing very small monodisperse silica projectiles (470 nm - 10 μm) at approximately 6 km s-1. The results show an unexpected departure from linear relationship between 1 and 10 μm. We collated crater measurement data and, where applicable, impactor residue data for 596 craters gathered during the post-mission preliminary examination (PE) phase. Using the new calibration, we recalculate the size of the particle responsible for each crater and hence reinterpret the cometary dust size distribution. We find a greater flux of small particles than previously reported. From crater morphology and residue composition of a sub-set of craters, the internal structure and dimensions of the fine dust particles is inferred and a "maximum-size" distribution for the sub-grains composing aggregate particles is obtained. The size distribution of the small particles derived directly from the measured craters peaks at ~175 nm, but if this is corrected to allow for aggregate grains, the peak in sub-grain sizes is at <100 nm.

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