ANALYSIS BY STXM AND PTYCHOGRAPHY. A. L. Butterworth1, A. J. Westphal1, Z. Gainsforth1, D. Shapiro2, Y. Yu2, D. Zevin1, R. Lettieri1, W. Marchant1, A. Ardizzone3, M. Capraro3, T. Yahnke3, C. P. Gonzalez4, R. Bastien4 and M. Zolensky4. 1University of California, Berkeley, Space Sciences Laboratory, 7 Gauss Way, Berkeley, CA, 94720-7450, 2Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, 3Stardust@Home volunteers, 4KT NASA Johnson Space Center, Houston, TX 77058, USA.

Introduction: The Interstellar Dust Collector on the Stardust spacecraft was exposed to the interstellar dust stream for ∼200 days in two intervals prior to the encounter of the spacecraft with comet Wild 2. The ∼0.1 m collector consisted of silica aerogel tiles (∼85% of the area), and Al foils (∼15% of the area). The collecting media are complementary: aerogel is superior in preserving impactor mineralogy and in preserving trajectory information, and foils are superior in ease of isotopic analysis, in being free of the major-rock forming element Si, and in allowing identification of smaller impactors. A consortium organized to identify and characterize impacts on the Interstellar Collector, the Interstellar Preliminary Examination (ISPE)[1], developed techniques to identify and characterize impacts in aerogel and foil collectors. In the aerogel tiles, impacts could be identified through optical microscopy. We and our colleagues developed an automated optical microscope to collect optical imagery, and we developed an online “virtual microscope” to enable citizen scientists to search for tracks, in a project called Stardust@home[2]. More than 30,000 citizen scientists have collectively carried out more than 10 searches, and this ongoing project has resulted in the indentification of >200 impactors in the aerogel tiles. Both individual and ensemble-wide detection efficiency was measured using calibration images, which constituted approximately 20% of the images. (The volunteer “dusters” were aware of the calibration images, but the identify of specific calibration images was hidden. The use of calibration images also enabled the generation of a score for each user, which served as a motivator, and was publicly posted on the Stardust@home website.) Among 71 tracks identified during the ISPE, three had characteristics consistent with an interstellar origin[4]. In contrast with tracks in the aerogel tiles, impacts in the foils result in craters that are too small to resolve optically, so the ISPE developed automated scanning procedures for Scanning Electron Microscopy[5]. The images were searched by eye and using automated algorithms[3], and as a result 25 impacts were identified, four of which were consistent in their characteristics with an interstellar origin[4]. As a complementary approach to searching within research groups and to automated identification, we have developed a new and separate instantiation of Stardust@home for the identification of craters in SEM images of the Al foils. Methods: The foils were removed from the Stardust collector using a double-bladed “pizza cutter”, resulting in strips ∼2mm wide and 15 mm or 30 mm long, depending on whether the foil was adjacent to the short or long side of the aerogel block. We used mounts developed for the purpose during ISPE, which stretched the foils flat to minimize focusing problems. We followed contamination control protocols[5], to avoid excessive carbon deposition on the foils during scanning. The foils were scanned at NRL with a FEI Nova 600 FIB-SEM at 40nm/pixel. We scanned foils I1020W and I1126N, resulting in 24405 total images, which were split and compressed, and 219, 555 images were uploaded to the Amazon Storage for access to the foils search Virtual Microscope. Figure 1 shows a typical field of view.

The NASA Stardust mission used silica aerogel slabs to slowly decelerate and capture impinging cosmic dust particles for return to Earth. During this process, impact tracks are generated along the trajectory of the particle into the aerogel. It is believed that the morphology and dimensions of these tracks, together with the state of captured grains at track termini, may be linked to the size, velocity, and density of the impacting cosmic dust grain. Here, we present the results of laboratory hypervelocity impact experiments, during which cosmic dust analog particles (diameters of between 0.2 and 0.4 mu m), composed of olivine, orthopyroxene, or an organic polymer, were accelerated onto Stardust flight-spare low-density (approximately 0.01 g cm(-3)) silica aerogel. The impact velocities (3-21 km s(-1)) were chosen to simulate the range of velocities expected during Stardust's interstellar dust (ISD) collection phases. Track lengths and widths, together with the success of particle capture, are analyzed as functions of impact velocity and particle composition, density, and size. Captured terminal particles from low-density organic projectiles become undetectable at lower velocities than those from similarly sized, denser mineral particles, which are still detectable (although substantially altered by the impact process) at 15 km s(-1). The survival of these terminal particles, together with the track dimensions obtained during low impact speed capture of small grains in the laboratory, indicates that two of the three best Stardust candidate extraterrestrial grains were actually captured at speeds much lower than predicted. Track length and diameters are, in general, more sensitive to impact velocities than previously expected, which makes tracks of particles with diameters of 0.4 mu m and below hard to identify at low capture speeds (<10 km s(-1)). Therefore, although captured intact, the majority of the interstellar dust grains returned to Earth by Stardust remain to be found.

Although synchrotron radiation X-Ray fluorescence (SR-XRF) is among the least destructive analysis methods applied to rare extraterrestrial grains, we have observed radiation damage effects following high flux synchrotron analyses. Track 30 of the IS collector of the Stardust mission , containing 2 candidates dubbed Orion and Sirius was analyzed at ESRF, France, on beamlines ID13 and ID22NI by nano-XRF/XRD scanning methods. Beam damage effects were noticed on both samples and a quantitative analysis of their irradiation history was established , allowing us to propose new experimental protocols as well as fluence limits, minimizing such effects in the future. The purpose of this study is to present these facts, analyze potential damage mechanisms and offer alternatives.

Here we describe the critical role that synchrotron X-ray and infrared microprobes are playing in the search for interstellar dust in the Stardust Interstellar Dust Collector (SIDC). The samples under examination are submicron particles trapped in low-density aerogel. We have found that the spatial resolution, energy range, and flux capabilities of the FTIR beamlines 1.4.3, ALS, and U2B, NSLS; the XRF microprobes ID13 and ID22NI, ESRF and 2-ID-D, APS; and the STXM beamline 11.0.2, ALS are ideally suited for studying these tiny returned samples. Using nondestructive, coordinated analyses at these microprobes, we have been able to eliminate most candidates as likely samples of interstellar dust. This in itself is a major accomplishment, since the analysis of these tiny samples is technically extremely challenging.

STARDUST AEROGEL INTERSTELLAR DUST COLLECTOR A. J. Westphal, C. C. Allen, S. Armes, S. Bajt, A. D. Ball, R. Bastien, H. Bechtel, J. Borg, F. E. Brenker, J. C. Bridges, D. E. Brownlee, M. J. Burchell, M. Burghammer, A. L. Butterworth, R. Chater, P. Cloetens, G. Cody, A. M. Davis, T. Ferroir, C. Floss, G. F. Flynn, D. Frank, Z. Gainsforth, E. Grun, P. R. Heck, J. Hillier, P. Hoppe, F. Horz, L. Howard, G. Howe, B. Hudson, G. R. Huss, J. Huth, A. T. Kearsley, B. Lai, M. Landgraf, L. Lemelle, J. Leitner, H. Leroux, R. Lettieri, W. Marchant, L. Nittler, R. Ogliore, M. C. Price, F. Postberg, S. A. Sandford, S. Schmitz, G. Silversmit, A. S. Simionovici, R. Srama, F. Stadermann, T. Stephan, R. M. Stroud, S. Sutton, R. Toucoulou, M. Trieloff, J. Trigo-Rodriguez, P. Tsou, A. Tsuchiyama, T. Tyliczszak, B. Vekemans, L. Vincze, J. Warren, M. E. Zolensky, >28,800 Stardust@home dusters. Affiliations are listed at http://ssl.berkeley.edu/~westphal/ispe.

In January 2006 the Stardust sample return capsule returned to Earth bearing the first solid samples from a primitive solar system body, C omet 81P/Wild2, and a collector dedicated to the capture and return o f contemporary interstellar dust. Both collectors were approximately 0.1m(exp 2) in area and were composed of aerogel tiles (85% of the co llecting area) and aluminum foils. The Stardust Interstellar Dust Col lector (SIDC) was exposed to the interstellar dust stream for a total exposure factor of 20 m(exp 2-) day during two periods before the co metary encounter. The Stardust Interstellar Preliminary Examination ( ISPE) is a three-year effort to characterize the collection using no ndestructive techniques. The ISPE consists of six interdependent proj ects: (1) Candidate identification through automated digital microsco py and a massively distributed, calibrated search (2) Candidate extr action and photodocumentation (3) Characterization of candidates thro ugh synchrotronbased FourierTranform Infrared Spectroscopy (FTIR), S canning XRay Fluoresence Microscopy (SXRF), and Scanning Transmission Xray Microscopy (STXM) (4) Search for and analysis of craters in f oils through FESEM scanning, Auger Spectroscopy and synchrotronbased Photoemission Electron Microscopy (PEEM) (5) Modeling of interstell ar dust transport in the solar system (6) Laboratory simulations of h ypervelocity dust impacts into the collecting media

, Ronald K. Bastien , Anna L. Butterworth, Josh Von Korff , David Anderson, Bryan Mendez , Rastika Prasad , Nicole Kelley, David Frank, Robert Lettieri , Zack Gainsforth, Christopher J. Snead , Jack L. Warren , Michael E. Zolensky , 20064 Stardust@home “dusters” 3 , 1 Space Sciences Laboratory, University of California at Berkeley, Berkeley CA 94720, USA 2 KT NASA Johnson Space Center, Houston, TX 77058, USA 3 tardust@home volunteers located world-wide .