NRL Scientists Analyze Comet Wild 2 Samples

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Scientists at the Naval Research Laboratory have analyzed samples from Comet Wild 2, as part of NASA's Stardust mission, the first solid sample return mission since Apollo. Over one hundred scientists at various institutions participated in the preliminary analysis. NRL contributed to the Mineralogy and Petrology, Crater, Bulk Chemistry and Isotope analysis teams by studying the structure and composition of the comet samples using transmission electron microscopy (TEM).

The TEM studies carried out by Dr. Rhonda Stroud and Dr. Thomas Zega, from NRL's Materials Science and Technology Division, demonstrated that micrometer-sized Mg-rich silicates and nanoscale Fe-sulfides are primary components of the cometary material. The samples studied at NRL included both grains captured in aerogel (Figure 1) and aluminum foil (Figure 2). Comparative analysis of samples collected in each media is important for distinguishing the primary characteristics of the cometary grains from capture artifacts. To conduct these studies, the NRL researchers made use of world-class TEM and focused-ion-beam microscopy (FIB) facilities maintained by the Materials Science and Technology Division and the Nanoscience Institute. FIB techniques for extracting the micrometer-sized cometary residue from foil craters were developed at NRL with support from the NASA SRLIDAP program.

Stardust was the first NASA mission dedicated to exploring a comet. The mission robotically collected comet samples in deep space and returned them to earth. Stardust passed within 149 miles of Comet Wild 2 in January 2004. The spacecraft passed Comet Wild 2 at 13,000 mph, over six times faster than a speeding bullet. The thousands of comet particles were captured using a material called aerogel, which is a special type of foamed glass, made so lightweight that it is barely visible and almost floats in air. Most of the particles collected are smaller than the width of a human hair. To collect the comet samples, Stardust traveled two-billion miles to meet Comet Wild 2 and then another one-billion miles to get back home. The samples returned to Earth in January 2006, and the preliminary analysis was conducted until August. The preliminary results appear in the December 15 issue of Science. Continued analysis of more of the Stardust cometary samples is ongoing.

Fig. 1 Bright-field TEM image of a mineral assemblage from Comet Wild 2. Mg-rich silicates, such as pyroxene (MgSiO3; Px) are intermixed with amorphous silicates (Am) and nanoscale Fe-Ni sulfides (black arrowheads). The aerogel capture material occurs around the assemblage.

Figure 2 A crater in aluminum foil containing Wild 2 comet material. B. Scanning transmission electron micoscope image of an extracted slice of the crater. C. a Principal component map identifying the cometary material components: red = iron-nickel sulfide; blue= magnesium-silicate; yellow = aluminum foil capture medium; green = protective carbon coating.

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