Results are presented for the fragmentation of projectiles in laboratory experiments. 1.5 mm cubes and spheres of basalt and shale were impacted onto water at normal incidence and speeds from 0.39 to 6.13 km s(-1); corresponding to peak shock pressures 0.7-32 GPa. Projectile fragments were collected and measured (over 100,000 fragments in some impacts, at sizes down to 10 mu m). Power laws were fitted to the cumulative fragment size distributions and the evolution of the exponent vs. impact speed and peak shock pressure found. The gradient of each of these power laws increased with increasing impact speed/peak shock pressure. The percentage of the projectiles recovered in the impacts was found and used to estimate projectile remnant survival in different solar system impact scenarios at the mean impact speed appropriate to that scenario. For Pluto, the Moon and in the asteroid belt approximately 55%, 40% and 15%, respectively, of an impactor could survive and be recovered at an impact site. Finally, the catastrophic disruption energy densities of basalt and shale were measured and found to be 24 x 10(4)J kg(-1) and 9 x 10(4)J kg(-1), respectively, a factor of similar to 2.5 difference. These corresponded to peak shock pressures of 1 to 1.5 GPa (basalt), and 0.8 GPa (shale). This is for near normal-incidence impacts where tensile strength is dominant. For shallow angle impacts we suggest shear effects dominate, resulting in lower critical energy densities and peak shock pressures. We also determine a method to ascertain information about fragment sizes in solar system impact events using a known size of impactor. The results are used to predict projectile fragments sizes for the Veneneia and Rheasilvia crater forming impacts on Vesta, and similar impacts on Ceres. (C) 2018 The Authors. Published by Elsevier Inc.

The mineralogy of comet 81P/Wild 2 particles, collected in aerogel by the Stardust mission, has been determined using synchrotron Fe-K X-ray absorption spectroscopy with insitu transmission XRD and X-ray fluorescence, plus complementary microRaman analyses. Our investigation focuses on the terminal grains of eight Stardust tracks: C2112,4,170,0,0; C2045,2,176,0,0; C2045,3,177,0,0; C2045,4,178,0,0; C2065,4,187,0,0; C2098,4,188,0,0; C2119,4,189,0,0; and C2119,5,190,0,0. Three terminal grains have been identified as near pure magnetite Fe3O4. The presence of magnetite shows affinities between the Wild 2 mineral assemblage and carbonaceous chondrites, and probably resulted from hydrothermal alteration of the coexisting FeNi and ferromagnesian silicates in the cometary parent body. In order to further explore this hypothesis, powdered material from a CR2 meteorite (NWA 10256) was shot into the aerogel at 6.1kms(-1), using a light-gas gun, and keystones were then prepared in the same way as the Stardust keystones. Using similar analysis techniques to the eight Stardust tracks, a CR2 magnetite terminal grain establishes the likelihood of preserving magnetite during capture in silica aerogel.

J. L. MacArthur, J. C. Bridges, L. J. Hicks, M. C. Price, J. E. Wickham-Eade, M. J. Burchell, G.M. Hansford, A. L. Butterworth. University of Leicester, UK, jm650@le.ac.uk. University of Kent, UK. University of California at Berkeley, CA, USA. Introduction: The Stardust mission to Comet Wild2 has provided unaltered cometary material for study in the form of terminal grains. Investigation of these has shown similarities with carbonaceous chondrites including chondrule and CAI fragments [1-3] and the range of olivine Fo3-100 and pyroxene compositions [4-5]. Iron oxides, particularly magnetite in terminal grains, have also been identified in Stardust keystones which by analogy with carbonaceous chondrites is evidence for water-rock reaction on the parent body [6-8]. In order to explore the closest chondrite analogues to Wild2, we have shot characterised CR2 and CV3 powders into aereogel and then analysed terminal grains using the same techniques as for Stardust grains [9]. Methods: A thin section and a 25 200 μm powder from Northwest Africa (NWA) 4502 (CV3) and NWA 10256 (CR2) were characterised using SEM-EDX. The powders were also examined with Raman spectroscopy then fired into aerogel of density 25-55 mg/cm at speeds of 6.1-6.3 kms [10-11]. The impact tracks were made into keystones, which were then analysed using Diamond synchrotron Fe-K XANES and SR-XRD, with a 3 m spot size. A Stardust terminal grain, track C2009,20,77,1,6 (track #77), was examined with EDX-STEM for comparison. Results: NWA 4502 has 38% matrix, 14% CAIs and 48% other chondrules. The chondrules contain pyroxene En50-98Wo0-34Fs0-2 and olivine Fo66-100, the matrix has FeNi sulfide, metal and olivine Fo36-50, pyroxene En64Wo36. NWA 10256 has 42% matrix, 58% chondrules. Pyroxene En27-99Wo0-6Fs1-67 and olivine Fo66-99 are found in chondrules, with olivine Fo41-81, Fe-sulfides, FeNi metal, and Fe-oxides in the matrix. Track #77 shows olivine Fo71-74. XANES and XRD: Two CR2 terminal grains (TG1, TG2) and a CV3 terminal grain (TG2) showed an Fe-K XANES match with olivine (Fig. 1A), with closely matching pre-edge and edge energies to the San Carlos standard and an olivine in the CR2 thin section. XRD confirmed this identification (Fig. 1B) with the 6 most intense peaks showing a close match to forsterite [12]. The unit cell dimensions for olivine increase as the Mg# decreases. Calculating unit cell dimensions from the XRD data and comparing with dimensions from the ICDD [12], then applying methods of [13] we calculate that the CR2 TG1 and TG2 are Fo59.6 and the CV3 TG2 is Fo43.4, with experimental error of ±1. Another of the CV3 grains showed a good match with magnetite with Fe-K XANES and XRD, similar to previous results [6].

Introduction: Terminal grains in Stardust keystones provide the most pristine cometary material for study collected from Comet Wild2. Investigation of these particles has revealed increasing evidence of similarities between the Wild2 constituents and carbonaceous chondrites. Such evidence includes Al-rich, and FeMg chondrule fragments and particles similar to late-forming chondrules in CR chondrites [1,2], as well as Al-rich and Ti-bearing clinopyroxenes with Mg-Al spinel consistent with CAI’s [3,4]. Another feature of the Wild2 particles is the iron oxides identified in Stardust keystones, suggesting further similarities with carbonaceous chondrites [5-7]. Magnetite and magnetitehematite mixtures [6,8] have been found along track walls and magnetite has been found in terminal grains [5,9,10], which are consistent with carbonaceous chondrite matrix material. The magnetite is assumed to be the result of the hydrous alteration of co-existing ferromagnesian minerals, also abundant in the Wild2 grains [10]. In order to identify the closest chondrite analogues for Wild2 we are studying mineralogically characterised CR2 and CV3 powders shot into aerogel, and then prepared as keystones, analogous to the way Wild2 samples were captured by Stardust and subsequently analysed. Methods: Polished sections were made from NWA 4502 (CV3) and NWA 10256 (CR2). Interior parts of each sample, away from the crust, were ground into a powder with grain size 25 200 μm. Half of the powders were fired into aerogel of density gradient 25-55 mg/cm at speeds of 6.1-6.3 kms using the University of Kent light gas gun [11] while the other half were made into polished blocks for further analysis. The sections and blocks were characterised using a Phillips XL30 ESEM with Oxford INCA 350 EDX system at the University of Leicester. Raman analyses were made at the University of Kent [6] and keystones at the University of Berkeley [12]. Results: Image analysis of NWA 4502 showed 38% matrix, 14% CAI’s and 48% chondrules. Pyroxene is En5098Wo0-34Fs0-2 and olivine Fo66-100 within chondrules; with olivine Fo36-50, Fe-Ni-metals, Fe oxide and sulfides present in the matrix. NWA 10256 was found to have 42% matrix including sulfides, metal and Fe oxide. Chondrule pyroxene and olivine are En89-98Wo0-1Fs1-10, Fo91-99, with more fayalitic olivine Fo34-40 present in the matrix. Raman Analyses of the Tracks. Hematite, enstatite and forsterite have been identified in three of the aerogel tracks of the CR2 powdered sample.