¹Anna L. Butterworth et al. (>10)*
¹Space Sciences Laboratory, University of California, Berkeley, California, USA

We report the quantitative characterization by synchrotron soft X-ray spectroscopy of 31 potential impact features in the aerogel capture medium of the Stardust Interstellar Dust Collector. Samples were analyzed in aerogel by acquiring high spatial resolution maps and high energy-resolution spectra of major rock-forming elements Mg, Al, Si, Fe, and others. We developed diagnostic screening tests to reject spacecraft secondary ejecta and terrestrial contaminants from further consideration as interstellar dust candidates. The results support an extraterrestrial origin for three interstellar candidates: I1043,1,30 (Orion) is a 3 pg particle with Mg-spinel, forsterite, and an iron-bearing phase. I1047,1,34 (Hylabrook) is a 4 pg particle comprising an olivine core surrounded by low-density, amorphous Mg-silicate and amorphous Fe, Cr, and Mn phases. I1003,1,40 (Sorok) has the track morphology of a high-speed impact, but contains no detectable residue that is convincingly distinguishable from the background aerogel. Twenty-two samples with an anthropogenic origin were rejected, including four secondary ejecta from impacts on the Stardust spacecraft aft solar panels, nine ejecta from secondary impacts on the Stardust Sample Return Capsule, and nine contaminants lacking evidence of an impact. Other samples in the collection included I1029,1,6, which contained surviving solar system impactor material. Four samples remained ambiguous: I1006,2,18, I1044,2,32, and I1092,2,38 were too dense for analysis, and we did not detect an intact projectile in I1044,3,33. We detected no radiation effects from the synchrotron soft X-ray analyses; however, we recorded the effects of synchrotron hard X-ray radiation on I1043,1,30 and I1047,1,34.

Reference
Butterworth AL et al. (2014) Stardust Interstellar Preliminary Examination IV: Scanning transmission X-ray microscopy analyses of impact features in the Stardust Interstellar Dust Collector. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12220]

Published by arrangement with John Wiley & Sons

¹Andrew J Westphal et al. (>10)*
¹Space Sciences Laboratory, University of California, Berkeley, California, USA
*Find the extensive, full author and affiliation list on the publishers website

With the discovery of bona fide extraterrestrial materials in the Stardust Interstellar Dust Collector, NASA now has a fundamentally new returned sample collection, after the Apollo, Antarctic meteorite, Cosmic Dust, Genesis, Stardust Cometary, Hayabusa, and Exposed Space Hardware samples. Here, and in companion papers in this volume, we present the results from the Preliminary Examination of this collection, the Stardust Interstellar Preliminary Examination (ISPE). We found extraterrestrial materials in two tracks in aerogel whose trajectories and morphology are consistent with an origin in the interstellar dust stream, and in residues in four impacts in the aluminum foil collectors. While the preponderance of evidence, described in detail in companion papers in this volume, points toward an interstellar origin for some of these particles, alternative origins have not yet been eliminated, and definitive tests through isotopic analyses were not allowed under the terms of the ISPE. In this summary, we answer the central questions of the ISPE: How many tracks in the collector are consistent in their morphology and trajectory with interstellar particles? How many of these potential tracks are consistent with real interstellar particles, based on chemical analysis? Conversely, what fraction of candidates are consistent with either a secondary or interplanetary origin? What is the mass distribution of these particles, and what is their state? Are they particulate or diffuse? Is there any crystalline material? How many detectable impact craters (>100 nm) are there in the foils, and what is their size distribution? How many of these craters have analyzable residue that is consistent with extraterrestrial material? And finally, can craters from secondaries be recognized through crater morphology (e.g., ellipticity)?

Reference
Westphal AJ et al. (2014) Final reports of the Stardust Interstellar Preliminary Examination. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12221]

Published by arrangement with John Wiley & Sons

¹Frank E. Brenker et al. (>10)*
¹Goethe University Frankfurt, Frankfurt am Main, Germany
*Find the extensive, full author and affiliation list on the publishers Website

Here, we report analyses by synchrotron X-ray fluorescence microscopy of the elemental composition of eight candidate impact features extracted from the Stardust Interstellar Dust Collector (SIDC). Six of the features were unambiguous tracks, and two were crater-like features. Five of the tracks are so-called “midnight” tracks—that is, they had trajectories consistent with an origin either in the interstellar dust stream or as secondaries from impacts on the Sample Return Capsule (SRC). In a companion paper reporting synchrotron X-ray diffraction analyses of ISPE candidates, we show that two of these particles contain natural crystalline materials: the terminal particle of track 30 contains olivine and spinel, and the terminal particle of track 34 contains olivine. Here, we show that the terminal particle of track 30, Orion, shows elemental abundances, normalized to Fe, that are close to CI values, and a complex, fine-grained structure. The terminal particle of track 34, Hylabrook, shows abundances that deviate strongly from CI, but shows little fine structure and is nearly homogenous. The terminal particles of other midnight tracks, 29 and 37, had heavy element abundances below detection threshold. A third, track 28, showed a composition inconsistent with an extraterrestrial origin, but also inconsistent with known spacecraft materials. A sixth track, with a trajectory consistent with secondary ejecta from an impact on one of the spacecraft solar panels, contains abundant Ce and Zn. This is consistent with the known composition of the glass covering the solar panel. Neither crater-like feature is likely to be associated with extraterrestrial materials. We also analyzed blank aerogel samples to characterize background and variability between aerogel tiles. We found significant differences in contamination levels and compositions, emphasizing the need for local background subtraction for accurate quantification.

Reference
Brenker FE et al. (2014) Stardust Interstellar Preliminary Examination V: XRF analyses of interstellar dust candidates at ESRF ID13. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12206]

Published by arrangement with John Wiley & Sons

¹Andrew J. Westphal et al. (>10)*
¹Space Sciences Laboratory, U.C. Berkeley, Berkeley, California, USA
*Find the extensive, full author and affiliation list on the publishers Website

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@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.

Reference
Westphal AJ et al. (2014) Stardust Interstellar Preliminary Examination I: Identification of tracks in aerogel. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12168]

Published by arrangement with John Wiley & Sons

¹Alexandre S. Simionovici et al. (>10)*
¹Institut des Sciences de la Terre, Observatoire des Sciences de l’Univers de Grenoble, Grenoble, France
*Find the extensive, full author and affiliation list on the publishers Website

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.

Reference
Simionovici AS et al. (2014) Stardust Interstellar Preliminary Examination VI: Quantitative elemental analysis by synchrotron X-ray fluorescence nanoimaging of eight impact features in aerogel. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12208]

Published by arrangement with John Wiley & Sons

^1,5M.D. Hopkins,^1,2,3,5S.J. Mojzsis, ^4,5W.F. Bottke, ^5,6O. Abramov
¹Department of Geological Sciences, University of Colorado, UCB 399, 2200 Colorado Avenue, Boulder, CO 80309-0399 USA
²Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon & Université Claude Bernard Lyon 1, CNRS UMR 5276, 2 rue Raphaël DuBois, Bât. Geode 3e, 69622 Villeurbanne, France
³Institute for Geological and Geochemical Research, RCAES, Hungarian Academy of Sciences, Budapest, Budaörsi ut 45, H-1112, Hungary
⁴Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302 USA
⁵Center for Lunar Origin and Evolution (CLOE), NASA Lunar Science Institute
⁶United States Geological Survey, Astrogeology Research Program, 2255 N. Gemini Dr., Flagstaff, AZ 86001 USA

Meteoritic zircons are rare, but some are documented to occur in asteroidal meteorites, including those of the howardite-eucrite-diogenite (HED) achondrite clan (Rubin, 1997). The HEDs are widely considered to originate from the asteroid 4 Vesta. Vesta and the other large main belt asteroids record an early bombardment history. To explore this record, we describe sub-micrometer distributions of trace elements (U, Th) and 235,238U-207,206Pb ages from four zircons (>7-40 μm ∅) separated from bulk samples of the brecciated eucrite Millbillillie. Ultra-high resolution (∼100 nm) ion microprobe depth profiles reveal different zircon age domains correlative to mineral chemistry and to possible impact scenarios. Our new U-Pb zircon geochronology shows that Vesta’s crust solidified within a few million years of solar system formation (4561±13 Ma), in good agreement with previous work (e.g. Carlson and Lugmair, 2000). Younger zircon age domains (ca. 4530 Ma) also record crustal processes, but these are interpreted to be exogenous because they are well after the effective extinction of 26Al (t1/2= 0.72 Myr). An origin via impact-resetting was evaluated with a suite of analytical impact models. Output shows that if a single impactor was responsible for the ca. 4530 Ma zircon ages, it had to have been ⩾10 km in diameter and at high enough velocity (>5 km s-1) to account for the thermal field required to re-set U-Pb ages. Such an impact would have penetrated at least 10 km into Vesta’s crust. Later events at ca. 4200 Ma are documented in HED apatite 235,238U-207,206Pb age ( Zhou et al., 2011) and 40-39Ar age spectra ( Bogard, 2011). Yet younger ages, including those coincident with the Late Heavy Bombardment (LHB; ca. 3900 Ma), are absent from Millbillillie zircon. This is attributable to primordial changes to the velocity distributions of impactors in the asteroid belt, and differences in mineral closure temperatures (Tc zircon >> apatite).

Reference
Hopkins MD, Mojzsis SJ, Bottke WF, Abramov O (2014) Micrometer-scale U-Pb age domains in eucrite zircons, impact re-setting, and the thermal history of the HED parent Body. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.025]

Copyright Elsevier

¹Andrew S. Rivkin,²Erik Asphaug,³William F. Bottke
¹Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Rd. Laurel, MD 20723
²Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287-6004
³Southwest Research Institute, 1050 Walnut St., Suite 300 Boulder, CO 80302

Ceres is unusual among large (>250 km) asteroids in lacking a dynamical family. We explore possible explanations, noting that its particularly large size and the ubiquity of families associated with other large asteroids makes avoidance of a sufficiently-sized collision by chance exceedingly unlikely. Current models of Ceres’ thermal history and interior structure favor a differentiated object with an icy near-surface covered by an ∼∼0.1-1 km lag deposit, which could result in a collisional family of diverse, predominately icy bodies. We predict that sublimation of an icy Ceres family would occur on timescales of hundreds of millions of years, much shorter than the history of the solar system. Sublimation on a Ceres family body would be aided by a low non-ice fraction and a high average temperature, both of which would inhibit lag deposit development. Because there seems to be no likely mechanism for removing a rocky Ceres family, and because the formation of a Ceres family of some kind seems nearly statistically inevitable, the lack of a Ceres family is indirect but independent evidence for Ceres’ differentiation.
All of the other large asteroids lacking dynamical families (704 Interamnia, 52 Europa, and 65 Cybele) have spectral properties similar to Ceres, or otherwise suggesting ice at their surfaces. While other large asteroids with similar spectral properties do have families (24 Themis, 10 Hygiea, 31 Euphrosyne), their families are not well understood, particularly Hygiea.

Reference
Rivkin AS, Asphaug E, Bottke WF (2014) The Case of the Missing Ceres Family. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.007]

Copyright Elsevier

¹Adam Culka, ¹Filip Košek, ¹Petr Drahota, ¹Jan Jehlička
¹Charles University in Prague, Institute of Geochemistry, Mineralogy and Mineral Resources, Albertov 6, 12843 Prague, Czech Republic

The presence of sulfates of different hydration states, specifically magnesium sulfates, has been firmly established on Mars from data acquired by both orbital and in-situ measurements. A lander mission typically involves a variety of instruments capable of performing a wide range of experiments from mineralogical tasks to the search for traces of life. It is clear from ongoing research that Raman spectroscopy can cover all of these tasks, and it has already been decided that future mission to Mars will employ a miniature Raman spectrometer. In this paper we report and discuss the Raman spectra of various sulfate minerals, with an emphasis on the magnesium sulfates. These were acquired by a hand-held Raman instrument, using the presently uncommon 532 nm excitation, the wavelength that is planned for the ESA lander mission. A sufficient quality of spectra were obtained with reasonably low spectral acquisition times, and the characteristic shift of the sulfate ν1 band in the MgSO4·n(H2O) minerals was confirmed. This was used for the unambiguous identification of magnesium sulfates of different hydration states. The present testing has confirmed the good performance of the handheld instrumentation for discrimination of structurally similar sulfates of relevance for Mars studies. This step has been proposed as the basis for subsequent testing of this instrumentation under Earth-based but Mars-analogous conditions, even using currently existing miniaturized Raman prototypes.

Reference
Culka A, Košek F, Drahota P, Jehlička J (2014) Use of miniaturized Raman spectrometer for detection of sulfates of different hydration states – significance for Mars studies. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.017]

Copyright Elsevier

¹Alice J. DeSimone, ^1,2Thomas M. Orlando
¹School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400
²School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0400

H2O (ν = 0) and O(3PJ=2,1,0) desorbates were measured with resonance-enhanced multiphoton ionization following 157-nm irradiation of amorphous solid water (ASW) deposited on a lunar mare basalt. Both H2O photodesorption and O(3PJ) photodissociation products of ASW were studied in the attempt to better understand the competition between photodesorption and photodissociation of water in the condensed phase on a lunar surface. The oxygen atom time-of-flight (TOF) spectrum was measured as a function of spin-orbit state, H2O exposure, and 157-nm irradiation time. Maxwell-Boltzmann distributions with translational temperatures of 10,000 K, 1800 K, 400 K, and 89 K fit the four TOF components. For high H2O exposures, diffusion out of pores in the lunar substrate made a large portion of the O(3PJ) signal appear to be sub-thermal. Water depletion cross sections were measured at exposures between 0.1 and 10 Langmuir (1 L = 10-6 Torr s). These cross sections decreased with increasing coverage and matched previously measured cross sections from a lunar impact melt breccia. Additionally, non-resonant ionization was employed to detect vibrationally excited water indirectly through its fragments. The OH+ fragment of H2O (ν∗) and the O(3PJ) photodissociation product increased in intensity during prolonged irradiation as hydroxyl groups accumulated on the surface and then recombined. For an initial exposure of 5 L H2O, after reaching maximum signal, the cross sections for H2O (ν∗) and O(3P2) depletion were measured to be 1.2 x 10-19 cm2 and 6.7 x 10-20 cm2, respectively.

Reference
DeSimone AJ, Orlando TM (2014) H2O and O(3PJ) Photodesorption from Amorphous Solid Water Deposited on a Lunar Mare Basalt. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.023]

Copyright Elsevier

¹A. Shahar, ¹V.J. Hillgren, ²M.F. Horan, ^1,3J. Mesa-Garcia, c, ¹L.A. Kaufman, ²T.D. Mock
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, USA
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, USA
³Geology Department, Universidad EAFIT, Medellin, Colombia

A series of high pressure and temperature experiments were conducted to better constrain the Fe isotope fractionation during core-mantle differentiation in planetesimal and planetary bodies. Synthetic mixtures of oxides and metal having varying amounts of sulfur, approximating terrestrial and Martian compositions, were melted at 1-2 GPa and 1650°C. Iron isotopic equilibrium between the resulting metal and glass run products was verified for all experiments using the three-isotope technique. Purified Fe from metal and glass was analyzed by multiple-collector ICP-MS in high resolution mode. Iron alloy and silicate glass show a well-resolved Δ57Femetal-silicate of +0.12 ±0.04‰ in a sulfur-free system. Isotope fractionation increases with sulfur content to +0.43 ±0.03‰ at 18 wt.% sulfur in the metal. These results cannot be easily interpreted within the context of known Fe isotope ratios in most natural samples of planetary and asteroidal mantles and therefore suggest more complex processes affected the Fe isotope fractionation therein.

Reference
Shahar A, Hillgren VJ, Horan MF, Mesa-Garcia J, Kaufman LA, Mock TD (2014) Sulfur-controlled iron isotope fractionation experiments of core formation in planetary bodies. Geochimica et Cosmochinica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.08.011]

Copyright Elsevier