Comet 81p/wild 2: the Size Distribution of Finer (sub 10- Micrometre) Dust Collected by the Stardust Spacecraft

The fluence of dust particles < 10 micrometres 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 to 10 μm) at ~ 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.