Showing papers by "John Bridges published in 2007"

Petrographic classification of middle ordovician fossil meteorites from Sweden

Abstract: The maximum diameter of chromite (FeCr2O4) grains within L chondrites reflects the petrographic type of the sample. On the basis of our measurements of nine recent L chondrites, L3 chromite D-mac = 34-50 mu m, L4 = 87-150 mu m, L5 = 76-158 mu m, and L6 = 253-638 mu m. This variation reflects the crystallization of the chromite grains during parent body thermal metamorphism.We use this calibration to classify six fossil meteorites from the Middle Ordovician in Sweden as type 3 (or 4) to 6. The high flux of L chondrites at 470 Ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent L chondrite falls. The fossil meteorites have in places preserved recognizable chondrule textures, including porphyritic olivine, barred olivine, and radiating pyroxene. A large relict clast and fusion crust have also been tentatively identified in one fossil meteorite. Apart from chromite, all of the original meteorite minerals have been replaced by carbonate (and sheet silicate and sulfate) during diagenesis within the limestone host. The preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules. The range (0.4-0.6 mm) is consistent with measurements made in the same way on recent L chondrites. (Less)

Nanometer hypervelocity dust impacts in low Earth orbit

Abstract: [1] A study has been made of 60 nm thick aluminum films which have been exposed to the space environment outside of International Space Station (ISS) between 2002 and 2004. Field emission scanning electron microscopy has been used to provide high-resolution images of impact features less than 100 nm in diameter and of detailed impact morphologies at different spatial scales. Analysis of images reveals the incident directions and diameters of the impacting particles and allows separation of impacts at hypervelocity from those at lower velocities. This allows the separation of different particle populations. We find that most detected particles have been generated locally as the result of secondaries following larger impacts elsewhere on the ISS or as a result of docking maneuvers by the Progress supply module or by other spacecraft. There is also evidence for a population of dust particles incident at hypervelocity and with minimum diameters smaller than 10 nm. This particle flux of the dust particles is consistent with the expected flux of micrometeoroids with mass >10−18 g at 1 AU and may represent the first measurement of dust particles with such small masses in near-Earth space. Future experiments to measure the flux of nanometer-scale dust particles are proposed, both passive exposure cells for retrieval and also active detectors to provide access to dust/debris populations without the requirement for experiment retrieval.

Discovery of Non-random Spatial Distribution of Impacts in the Stardust Cometary Collector

Abstract: We report the discovery that impacts in the Stardust cometary collector are not distributed randomly in the collecting media, but appear to be clustered on scales smaller than {approx} 10 cm. We also report the discovery of at least two populations of oblique tracks. We evaluated several hypotheses that could explain the observations. No hypothesis was consistent with all the observations, but the preponderance of evidence points toward at least one impact on the central Whipple shield of the spacecraft as the origin of both clustering and low-angle oblique tracks. High-angle oblique tracks unambiguously originate from a non-cometary impact on the spacecraft bus just forward of the collector.

UV and Visible Wavelength Reflectance Spectroscopy of Aerogel and of Stardust Grains

Abstract: A new UV and visible reflectance microspectroscopy technique has been applied to cometary samples from the Stardust mission. The results have been compared with terrestrial minerals and aerogel.

SEM-EDS Analyses of Small Craters in Stardust Aluminum Foils: Implications for the Wild-2 Dust Distribution

Abstract: Aluminium foils were used on Stardust to stabilize the aerogel specimens in the modular collector tray. Part of these foils were fully exposed to the flux of cometary grains emanating from Wild 2. Because the exposed part of these foils had to be harvested before extraction of the aerogel, numerous foil strips some 1.7 mm wide and 13 or 33 mm long were generated during Stardusts's Preliminary Examination (PE). These strips are readily accommodated in their entirety in the sample chambers of modern SEMs, thus providing the opportunity to characterize in situ the size distribution and residue composition - employing EDS methods - of statistically more significant numbers of cometary dust particles compared to aerogel, the latter mandating extensive sample preparation. We describe here the analysis of nearly 300 impact craters and their implications for Wild 2 dust.

Stardust microcrater residue compositional groups

Abstract: Compositional groups are defined in residue from Stardust craters (1-9 Dc) by qualitative EDS. These compositional groups are being further studied by a FIB-SEM technique to determine representative residue compositions.