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Fine-grained Material Trapped in Stardust Track Walls

Introduction: Samples from comet Wild 2 have been collected in the silica aerogel collector of the Stardust spacecraft [1]. The hypervelocity impact of comet particles at 6.1 km/s has left tracks in the aero-gel which are terminated by one or several terminal particles. These terminal particles are usually coarse-grained silicate or iron-sulfide [2]. Abundant material is also present along the track walls as illustrated by synchrotron X-rays fluorescence spectroscopy [3,4]. This configuration suggests that the Wild 2 particles are made of poorly cohesive assemblages that have been broken and scattered during the impact into the aerogel. The study of material deposited along the track walls is important to understand the nature of the Wild 2 particles (structure of the aggregates, grain size and mineralogy of invidual components). Unfortunately the particles extracted from track walls are frequently strongly thermally modified. Their typical micro-structure consists of a vesicular, silica-rich glassy matrix containing a high density of Fe-Ni-S nanophases [5]. Most of these samples show CI-chondritic composition , including sulfur. The composition and micro-structure suggest that the comet fragments within the track walls originated from a fine-grained, loosely bound, dust aggregate, that were melted and mixed with molten aerogel, in contrast to the coarse-grained terminal particles which survived quite better to the capture process. In this work we describe the microstructure of mi-cro-tracks as studied by transmission electron microscope (TEM). The study of these micro-tracks has been rendered possible due to a new technique developed at the University of Washington for preparing ultra-thin sections for TEM [6]. Experimental: The studied sample originates from track 10. It has been prepared at the University of Washington [6]. A piece of aerogel was first extracted from the track wall, flattened between two glass slides, embedded into acrylic resin and then sliced by ultrami-crotomy. Since acrylic is soluble it has been removed after ultramicrotomy. The main advantage of this preparation method is that it preserves all pieces of collected samples in the aerogel medium (see [6] for a full description of the method and discussion about the potential applications). Four ultra-thin slices have been studied using a TEM FEI Tecnai G2-20 equipped with Energy Dispersive X-ray Spectroscopy (EDX) (see [4] for a full description of the analytical procedure).