In Situ Chemical Mapping and Analysis of Stardust Impact Tracks: Isolation of Cometary Signal

Physics

Scientific paper

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6008 Composition (1060), 6015 Dust, 6023 Comets: Dust Tails And Trails (6210), 6094 Instruments And Techniques, 6210 Comets (6023)

Scientific paper

Since the Stardust mission returned in January 2006 with particles from the coma of Comet P/Wild-2, several research groups at synchrotrons around the world have been involved in the Preliminary Examination of Stardust comet dust captured in silica aerogel cells. In situ analysis by x-ray microprobe of intact particle deceleration tracks, impact tracks, in the aerogel collection medium provides the chemical distribution of the cometary material. This includes the typically larger-grained and robust particles deposited at the track terminus as well as the more fine-grained and dispersed material that is deposited near the track entrance and is less accessible to traditional mineralogy studies by TEM. X-ray microprobes on synchrotron sources are powerful tools that can provide compositions of elements present only in trace quantities. Chemical maps and quantitative compositions of impact tracks and their terminal particles have been reported for the set of Stardust samples analyzed during the Preliminary Examination. A significant issue that arose in this process was how to distinguish the background contribution arising from contamination in the aerogel collection medium itself. These elements are present in addition to the ubiquitous Si in the aerogel, vary in concentration from cell to cell and include elements such as Fe and Ca that are relevant to the mineralogy and chemistry of the Stardust samples. Proper removal of this background is necessary for accurate results. We present an approach based on thresholds for distinguishing between the synchrotron x-ray fluorescence originating from the cometary material and that originating from the aerogel and contaminants. Results of this method applied to Stardust tracks are also presented. This approach may be used on any sample in a matrix containing elements of interest in the sample itself.

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