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

Abstract

Abstract-The fluence of dust particles <10 lm in diameter was recorded by impacts on aluminum foil of the NASA Stardust spacecraft during a close flyby of comet 81P ⁄ Wild 2 in 2004. Initial interpretation of craters for impactor particle dimensions and mass was based upon laboratory experimental simulations using projectiles less than >10 lm in diameter and the resulting linear relationship of projectile to crater diameter was extrapolated to smaller sizes. We now describe a new experimental calibration program firing very small monodisperse silica projectiles (470 nm-10 lm) at approximately 6 km s )1. The results show an unexpected departure from linear relationship between 1 and 10 lm. We collated crater measurement data and, where applicable, impactor residue data for 596 craters gathered during the postmission preliminary examination 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 subset of craters, the internal structure and dimensions of the fine dust particles are inferred and a ''maximumsize'' distribution for the subgrains composing aggregate particles is obtained. The size distribution of the small particles derived directly from the measured craters peaks at approximately 175 nm, but if this is corrected to allow for aggregate grains, the peak in subgrain sizes is at <100 nm.

Stardust Interstellar Preliminary Examination XI: Identification and elemental analysis of impact craters on Al foils from the Stardust Interstellar Dust Collector

“…By comparison with laboratory studies and computer simulations of normal incidence impacts of soda lime glass spheres, the velocity of the impacting dust grains can be estimated to be approximately 5 km s −1 (Price et al. , ). Of the four candidate interstellar craters, two showed double‐well crater floors, indicative of an impacting grain composed of at least two distinct subgrains, in agreement with the STEM‐EDX data for these craters showing both sulfide and silicate components.…”

“…Recent extension of the light‐gas gun studies to the sub‐μm projectile regime shows that the linear relationship between crater diameter ( D c ) and projectile diameter ( D p ) has a discontinuity at D p ≈ 2.4 μm (Price et al. ). For spherical particles with a density of 2.4 g cm −3 and diameter D p > 2.4 μm at impact speeds of 6 km s −1 , D c ~ 4.62 × D p .…”

Stardust Interstellar Preliminary Examination IX: High‐speed interstellar dust analog capture in Stardust flight‐spare aerogel

“…, ; Price et al. ). The size and speed of most ISD particles are beyond the capabilities of two‐stage light‐gas guns, which can confer a maximum velocity of approximately 7 km s −1 .…”

Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft

“…The diameter of these candidate interstellar craters ranges from 0.28 µm to 0.46 µm. The crater diameter (D c ) is a function of particle diameter (D p ), capture speed and density (26,27), with D c ~ 1.6D p for silica spheres impacting Al 1100 alloy at 6.1 km s -1 . Thus, the diameters of the particles that produced the craters range from ~0.2 to 0.3 µm.…”