PRISTINE STRATOSPHERIC COLLECTION OF COSMIC DUST. S. Messenger1, L. P. Keller1, K. Nakamu- ra-Messenger1,2, S.J. Clemett1,2 1Robert M. Walker Laboratory for Space Science, ARES, NASA JSC, 2101 NASA Parkway, Houston TX 77058, USA 2ESCG, Johnson Space Center, Houston TX 77058. ([email protected]) Introduction: Since 1981, NASA has routinely collected interplanetary dust particles (IDPs) in the stratosphere by inertial impact onto silicone oil-coated flat plate collectors deployed on the wings of high- altitude aircraft [1]. The highly viscous oil traps and localizes the particles, which can fragment during col- lection. Particles are removed from the collectors with a micromanipulator and washed of the oil using organic solvents, typically hexane or xylene. While silicone oil is an efficient collection medium, its use is problemat- ic. All IDPs are initially coated with this material (po- lydimethylsiloxane, n(CH ) SiO) and traces of oil may 3 2 remain after cleaning. The solvent rinse itself is also a concern as it likely removes indigenous organics from the particles. To avoid these issues, we used a polyure- thane foam substrate for the oil-free stratospheric col- Fig 1: W7262 photomosaic showing locations of lection of IDPs. particles A2 (top), A3 (bottom), and A5 (left). Experimental: We prepared an IDP collector by tron microscope (STEM) examination. Other particles attaching a ¼ inch thick sheet of polyurethane foam were first examined with a field-emission scanning (McMaster Carr) to a standard small-area IDP collector electron microscope (SEM), obtaining secondary elec- using a cyanoacrylate adhesive. Structurally, the foam tron images and energy dispersive x-ray (EDX) spec- consists of a three dimensional network of 40 m-thick tra. Several of these have been embedded in epoxy and strands and thin membranes forming ~400 m-wide examined by STEM. cells. Initial examination of the foam prior to flight Results & Discussion: Two of the particles (A2 & confirmed that the surface was clean and essentially A3) removed from the collector were confirmed to be devoid of visible particles. The collector (W7262) was IDPs and a third candidate IDP (A5) has been identi- flown for 8 hours in October 2006. fied. Particles A2 and A3 are fragile, fine grained clus- The collector was scanned for candidate IDPs using ter IDPs. On the collector, these appear as compact but a high-magnification stereo microscope in a clean incoherent masses surrounded by numerous room. Searching for particles on the foam collector micrometer-sized grains (Fig. 2). In both cases, the proved to be much more time consuming compared particles are contained within a single cell, with the with standard collectors. This is due to the fact that the surrounding cells devoid of particles. Several apparent depth of the cells is greater than the depth of field of contaminant metallic grains were identified on the col- the microscope and particles have less contrast against the substrate compared with standard IDP collectors. Relocating particles was also challenging due to the complex structure of the foam. We overcame these difficulties by first producing a high magnification pho- tomosaic of the entire collector (Fig. 1). Image stacks at differing focal points were obtained and converted into best-focus images using a commercial software package. Secondly, we used a computerized stage to record the positions of the particles. Candidate IDPs were first photo-documented and then removed with a micromanipulator, using dry glass needles. Several particles were directly embedded in Fig 2: Cluster IDP W7262 A3; particles are distri- epoxy and microtomed for scanning-transmission elec- buted throughout the interior of a single cell. are Mg-rich. A small anorthite grain was also observed in A2. The particles underwent moderate heating dur- ing atmospheric entry as evidenced by thin, disconti- nuous magnetite rims on some of the sulfides and GEMS grains, vesicular carbonaceous material and a lack of solar flare particle tracks. The chemical com- positions of 19 GEMS grains show heterogeneous compositions (Fig. 5), with mean Mg/Si, Ca/Si, Al/Si, Fig 3: SEM images of fragments of IDPs W7262 A2 & A3 Ni/S, and S/Si identical to our previous GEMS grain lector, perhaps originating from the aircraft. analyses [3]. The average Fe/Si ratio is somewhat low- SEM examination of fragments from A2 and A3 er than average GEMS grains. revealed the typical fine grained, fragile appearance of chondritic porous (CP) IDPs (Fig. 3). Energy disper- sive X-ray spectra obtained from several fragments of A2 and A3 are consistent with chondritic major ele- ment abundances. The candidate IDP A5 also appeared as a cluster of grains within a single cell, though coarser grained in comparison to A2 and A3. We examined one fragment of A5 by SEM, and this appeared to be mainly a single Fig 5: Left: Bright field TEM image of a cluster of GEMS grains in A2. Right: RGB (Mg,Fe,Si) composite elemental mineral grain with the EDX spectra consistent with Fe- map of GEMS grains obtained by STEM. rich olivine. A Ni peak was also observed, consistent While we have not observed any differences in the with an extraterrestrial origin. TEM work will be re- mineralogy of these IDPs compared with previously quired to further evaluate the origin of A5. studied IDPs attributable to the exposure to silicone oil Fragments of the cluster IDPs A2 (two samples) or hexane, organic studies will clearly benefit from this and A3 were embedded in low viscosity resin and thin type of dry collection. CP IDPs are very rich in isotop- sections were prepared using ultramicrotomy. Imaging ically anomalous organic matter that differs in some data, diffraction patterns and quantitative chemical respects from carbonaceous chondrite organics [4-7]. maps were obtained from microtome thin sections with These differences may reflect the fact that CP IDPs the JSC JEOL 2500 STEM equipped with a thin- have not been affected by hydrothermal processing. window EDX detector. However, it is also likely that soluble organic phases Both A2 (Fig. 4) and A3 have mineralogy typical of are removed from IDPs during the hexane rinse nor- CP IDPs: GEMS grains, FeNi sulfides, crystalline sili- mally used to remove silicone oil. We are planning cates (enstatite and forsterite), FeNi metal, equilibrated future organic and isotopic studies of these IDPs by aggregates and abundant carbonaceous material [2]. coordinated NanoSIMS and ultra-L2MS microprobe. The clinoenstatite platelets and forsterite single crystals As a collection substrate, the polyurethane foam performed well compared with silicone oil coated col- lectors. We have identified two and perhaps three clus- ter IDPs, with additional smaller particles on the col- lector remaining to be studied. If flown for the typical exposure time of IDP collectors, we could expect to collect at least 10 IDPs with this type of foam. Howev- er, other types of materials should be investigated for optimal cleanliness and collection efficiency. References: [1] Sandford S.A. (1987) Fund. Cosmic Phys. 12, 1-73. [2] Bradley J.P. (2004) in Treatise on Geochemistry Vol. 1: Meteorites, Comets, and Planets 689 [3] Keller L.P. & Messenger S. (2011) GCA 75, 5336 [4] Pizzarello S., Cooper G.W., & Flynn G.J. (2006) in Meteorites and the Early Solar System II, 625. [5] Clemett S.J. et al. (1993) Science 262, 721 [6] Fig 4: Bright field TEM image of a section of A2. The mineral grains and GEMS are contained within vesicular S. Messenger (2000) Nature 404, 968 [7] Floss C. et carbonaceous material (dark grey). al. (2004) Science 303, 1355