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Postberg F.,University of Heidelberg | Hillier J.K.,University of Heidelberg | Armes S.P.,University of Sheffield | Bugiel S.,Max Planck Institute for Nuclear Physics | And 68 more authors.
Meteoritics and Planetary Science | Year: 2014

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 μm), 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 μm 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. © The Meteoritical Society, 2014.

Flynn G.J.,SUNY Plattsburgh Plattsburgh | Sutton S.R.,Argonne National Laboratory | Lai B.,Argonne National Laboratory | Wirick S.,University of Chicago | And 65 more authors.
Meteoritics and Planetary Science | Year: 2014

The NASA Stardust spacecraft exposed an aerogel collector to the interstellar dust passing through the solar system. We performed X-ray fluorescence element mapping and abundance measurements, for elements 19 ≤ Z ≤ 30, on six "interstellar candidates," potential interstellar impacts identified by Stardust at Home and extracted for analyses in picokeystones. One, I1044,3,33, showed no element hot-spots within the designated search area. However, we identified a nearby surface feature, consistent with the impact of a weak, high-speed particle having an approximately chondritic (CI) element abundance pattern, except for factor-of-ten enrichments in K and Zn and an S depletion. This hot-spot, containing approximately 10 fg of Fe, corresponds to an approximately 350 nm chondritic particle, small enough to be missed by Stardust at Home, indicating that other techniques may be necessary to identify all interstellar candidates. Only one interstellar candidate, I1004,1,2, showed a track. The terminal particle has large enrichments in S, Ti, Cr, Mn, Ni, Cu, and Zn relative to Fe-normalized CI values. It has high Al/Fe, but does not match the Ni/Fe range measured for samples of Al-deck material from the Stardust sample return capsule, which was within the field-of-view of the interstellar collector. A third interstellar candidate, I1075,1,25, showed an Al-rich surface feature that has a composition generally consistent with the Al-deck material, suggesting that it is a secondary particle. The other three interstellar candidates, I1001,1,16, I1001,2,17, and I1044,2,32, showed no impact features or tracks, but allowed assessment of submicron contamination in this aerogel, including Fe hot-spots having CI-like Ni/Fe ratios, complicating the search for CI-like interstellar/interplanetary dust. © The Meteoritical Society, 2014.

Simionovici A.S.,CNRS Institute of Earth Sciences | Lemelle L.,Ecole Normale Superieure de Lyon | Cloetens P.,European Synchrotron Radiation Facility | Sole V.A.,European Synchrotron Radiation Facility | And 63 more authors.
Meteoritics and Planetary Science | Year: 2014

Hard X-ray, quantitative, fluorescence elemental imaging was performed on the ID22NI nanoprobe and ID22 microprobe beam lines of the European Synchrotron Research facility (ESRF) in Grenoble, France, on eight interstellar candidate impact features in the framework of the NASA Stardust Interstellar Preliminary Examination (ISPE). Three features were unambiguous tracks, and the other five were identified as possible, but not definite, impact features. Overall, we produced an absolute quantification of elemental abundances in the 15 ≤ Z ≤ 30 range by means of corrections of the beam parameters, reference materials, and fundamental atomic parameters. Seven features were ruled out as interstellar dust candidates (ISDC) based on compositional arguments. One of the three tracks, I1043,1,30,0,0, contained, at the time of our analysis, two physically separated, micrometer-sized terminal particles, the most promising ISDCs, Orion and Sirius. We found that the Sirius particle was a fairly homogenous Ni-bearing particle and contained about 33 fg of distributed high-Z elements (Z > 12). Orion was a highly heterogeneous Fe-bearing particle and contained about 59 fg of heavy elements located in hundred nanometer phases, forming an irregular mantle that surrounded a low-Z core. X-ray diffraction (XRD) measurements revealed Sirius to be amorphous, whereas Orion contained partially crystalline material (Gainsforth et al. 2014). Within the mantle, one grain was relatively Fe-Ni-Mn-rich; other zones were relatively Mn-Cr-Ti-rich and may correspond to different spinel populations. For absolute quantification purposes, Orion was assigned to a mineralogical assemblage of forsterite, spinel, and an unknown Fe-bearing phase, while Sirius was most likely composed of an amorphous Mg-bearing material with minor Ni and Fe. Owing to its nearly chondritic abundances of the nonvolatile elements Ca, Ti, Co, and Ni with respect to Fe, in combination with the presence of olivine and spinel as inferred from XRD measurements, Orion had a high probability of being extraterrestrial in origin. © The Meteoritical Society, 2014.

Westphal A.J.,Space Science Laboratory Uc Berkeley Berkeley | Anderson D.,Space Science Laboratory Uc Berkeley Berkeley | Butterworth A.L.,Space Science Laboratory Uc Berkeley Berkeley | Frank D.R.,NASA | And 65 more authors.
Meteoritics and Planetary Science | Year: 2014

Here, we report the identification of 69 tracks in approximately 250 cm2 of aerogel collectors of the Stardust Interstellar Dust Collector. We identified these tracks through Stardust at home, a distributed internet-based virtual microscope and search engine, in which > 30,000 amateur scientists collectively performed >9 × 107 searches on approximately 106 fields of view. Using calibration images, we measured individual detection efficiency, and found that the individual detection efficiency for tracks > 2.5 μm in diameter was >0.6, and was >0.75 for tracks >3 μm in diameter. Because most fields of view were searched >30 times, these results could be combined to yield a theoretical detection efficiency near unity. The initial expectation was that interstellar dust would be captured at very high speed. The actual tracks discovered in the Stardust collector, however, were due to low-speed impacts, and were morphologically strongly distinct from the calibration images. As a result, the detection efficiency of these tracks was lower than detection efficiency of calibrations presented in training, testing, and ongoing calibration. Nevertheless, as calibration images based on low-speed impacts were added later in the project, detection efficiencies for low-speed tracks rose dramatically. We conclude that a massively distributed, calibrated search, with amateur collaborators, is an effective approach to the challenging problem of identification of tracks of hypervelocity projectiles captured in aerogel. © The Meteoritical Society, 2014.

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