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Bradley, J. P.

Publications and source records attributed to Bradley, J. P..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Analysis of Cosmic Spherule Candidates from the Kwajalein Micrometeorite Collection

The Kwajalein micrometeorite collection utilised high volume air samplers fitted with 5 micrometer laser-etched polycarbonate membrane filters to capture particles directly from the atmosphere. The filters were changed weekly over several months throughout 2011/12, providing the opportunity to investigate the contemporary flux of micrometeorites. We recently reported the results of our initial survey of cosmic spherule-like particles on several of these filters. We identified three main groups of particle based on bulk compositions: 1. Silicate spherules rich in Mg, Ca and Fe, 2. Silicate spherules rich in Al, Ca, K and/or Na and 3. Fe-rich spherules. Abundances appeared to change over time suggesting links with celestial activity (e.g. meteor showers), however, spherules similar to groups 2 and 3 can be produced by terrestrial and anthropogenic activity (e.g. volcanic microspherules exhibit similar compositions to group 2 spherules and metallic spherules similar to those of group 3 can be formed during fuel combustion). We are now studying the internal structures and chemistries of these spherules and comparing against cosmic spherules identified in other collections to confrim their origins and further contrain the contemporary micrometeorite flux. Particles are being picked, embedded in resin and polished through to reveal their interiors. Here we will describe our ongoing analyses of these particles via SEM. We will also introduce our new collection using this method that is currently being performed in the Antarctic.

Wozniakiewicz, P. J.↗
NASA NTRS2014

Investigating the Present Day Cosmic Dust Flux at the Earth's Surface: Initial Results from the Kwajalein Micrometeorite Collection

Examination of impact craters on the Long Duration Exposure Facility satellite indicate a present day micrometeoroid flux of approx. 30,000 tonnes [1 after 2]. But what portion of this material arrives at the Earth's surface as micrometeorites? Studies of available micrometeorite collections from deep sea sediments [e.g. 3], Greenland blue ice [e.g. 4] and the South Pole water well [e.g. 1] may be complicated by terrestrial weathering and, in some cases, collection bias (magnetic separation for deep sea sediments) and poorly constrained ages. We have recently set up a micrometeorite collection station on Kwajalein Island in the Republic of the Marshall Islands in the Pacific Ocean, using high volume air samplers to collect particles directly from the atmosphere. By collecting in this way, the terrestrial age of the particles is known, the weathering they experience is minimal, and we are able to constrain particle arrival times. Collecting at this location also exploits the considerably reduced anthropogenic background [5]. Method: High volume air samplers were installed on top of the two-story airport building on Kwajalein. These were fitted with polycarbonate membrane filters with 5μm diameter perforations. The flow rates were set to 0.5m3/min, and filters were changed once a week. After collection, filters were washed to remove salt and concentrate particles [see 5] in preparation for analysis by SEM. Results and Discussion: A selection of filters have been prepared and surveyed. Due to their ease of identification our initial investigations have focused on particles resembling cosmic spherules. The spheres can be divided into three main groups: 1. Silicate spherules rich in Al, Ca, K and Na (to varying degrees), 2. Silicate spherules rich in Mg and Fe and 3. Fe-rich spherules. Group 1 spherules are often vesiculated and can occur as aggregates. They are similar in appearance and composition to volcanic microspheres [e.g. 6] and are thus likely terrestrial in origin (volcanic). Those of groups 2 and 3, however, typically exhibit quenched surface textures consistent with cosmic spherules. Initial results suggest there is significant variation in the abundance of these groups from filter to filter. Work is ongoing to fully characterize these spherules and to constrain their flux with time.

Wozniakiewicz, P. J.↗
NASA NTRS2014

Initial Results from the Kwajalein Micrometeorite Collections

Micrometeorites are constantly arriving at the Earth's surface, however, they are quickly diluted by the natural and anthropogenic back-ground dust. The successful collection of micromete-orites requires either the employment of a separation technique (e.g. using magnets to separate metal-bearing micrometeorites from deepsea sediments [e.g. 1,2] and dissolved pre-historic limestones and salts [e.g. 3,4]), or an approach that limits contamination by terrestrial dust (e.g. collecting from ice, snow and well water in polar regions - locations where the terrestrial dust flux is so low that micrometeorites repre-sent the major dust component [e.g. 5-7]). We have recently set up a micrometeorite collection station on Kwajalein Island in the Republic of the Marshall Is-lands in the Pacific Ocean, using high volume air samplers to collect particles directly from the atmosphere. Collecting at this location exploits the considerably reduced anthropogenic background; Kwajalein is >1000 miles from the nearest continent and for much of the year, trade winds blow from the northeast at 15 to 20 knots providing a continuous stream of oceanic aerosol for sampling. By collecting directly from the atmosphere, the terrestrial age of the particles, and hence weathering they experience, is minimal. We therefore anticipate that the Kwajalein col-lection may include particles that are highly susceptible to weathering and either not preserved well or not found at all in other collections. In addition, this collection method allows for particle arrival times to be constrained so that collections can be timed to correlate with celestial events (e.g. meteor showers). Here we describe the collections and their preparation and report on the initial results.

Wozniakiewicz, P. J.↗
NASA NTRS2007

Analysis of Cometary Dust Impact Residues in the Aluminum Foil Craters of Stardust

In January 2006, the sample return capsule from NASA s Stardust spacecraft successfully returned to Earth after its seven year mission to comet Wild-2. While the principal capture medium for comet dust was low-density graded silica aerogel, the 1100 series aluminum foil (approximately 100 m thick) which wrapped around the T6064 aluminum frame of the sample tray assembly (STA) contains micro-craters that constitute an additional repository for Wild-2 dust. Previous studies of similar craters on spacecraft surfaces, e.g. the Long Duration Exposure Facility (LDEF), have shown that impactor material can be preserved for elemental and mineralogical characterization, although the quantity of impact residue in Stardust craters far exceeds previous missions. The degree of shock-induced alteration experienced by the Wild-2 particles impacting on foil will generally be greater than for those captured in the low-density aerogel. However, even some of the residues found in LDEF craters showed not only survival of crystalline silicates but even their solar flare tracks, which are extremely fragile structures and anneal at around 600 C. Laboratory hypervelocity experiments, using analogues of Wild-2 particles accelerated into flight-grade foils under conditions close to those of the actual encounter, showed retention of abundant projectile residues at the Stardust encounter velocity of 6.1 km/s. During the preliminary examination (PE) of the returned foils, using optical and electron microscopy studies, a diverse range in size and morphologies of micro-craters was identified. In this abstract we consider the state of residue preservation in a diverse range of craters with respect to their elemental composition and inferred mineralogy of the original projectiles.

Graham, G. A.↗
NASA NTRS2006

Focused Ion Beam Recovery of Hypervelocity Impact Residue in Experimental Craters on Metallic Foils

The Stardust sample return capsule will return to Earth in January 2006 with primitive debris collected from Comet 81P/Wild-2 during the fly-by encounter in 2004. In addition to the cometary particles embedded in low-density silica aerogel, there will be microcraters preserved in the Al foils (1100 series; 100 micrometers thick) that are wrapped around the sample tray assembly. Soda lime spheres (approximately 49 m in diameter) have been accelerated with a light-gas-gun into flight-grade Al foils at 6.35 km s(sup -1) to simulate the potential capture of cometary debris. The preserved crater penetrations have been analyzed using scanning electron microscopy (SEM) and x-ray energy dispersive spectroscopy (EDX) to locate and characterize remnants of the projectile material remaining within the craters. In addition, ion beam induced secondary electron imaging has proven particularly useful in identifying areas within the craters that contain residue material. Finally, high-precision focused ion beam (FIB) milling has been used to isolate and then extract an individual melt residue droplet from the interior wall of an impact penetration. This enabled further detailed elemental characterization, free from the background contamination of the Al foil substrate. The ability to recover pure melt residues using FIB will significantly extend the interpretations of the residue chemistry preserved in the Al foils returned by Stardust.

Graham, G. A.↗
NASA NTRS2005

Cooked GEMS - Insights into the Hot Origins of Crystalline Silicates in Circumstellar Disks and the Cold Origins of GEMS

The comparison of interstellar, circumstellar and primitive solar nebula silicates has led to a significant conundrum in the understanding of the nature of solid materials that begin the planet forming processes. Crystalline silicates are found in circumstellar regions around young stars and also evolved stars ejecting particles into the interstellar medium (ISM) but they are not seen in the interstellar medium itself, the source material for star and planet formation. Crystalline silicates are minor to major components of all known early solar system materials that have been examined as meteorites or interplanetary dust samples. The strong presence of Mg-rich crystalline silicates in Oort cloud comets and their minor presence in some Kuiper belt comets is also indicated by 11.2 m peak in approx. 10 microns "silicate" infrared feature. This evidence strongly indicates that Mg-rich crystalline silicates were abundant components of the solar nebula disk out to at least 10 AU, and present out to 30 AU.

Brownlee, D. E.↗
NASA NTRS2005

High-Resolution Electron Energy-Loss Spectroscopy (HREELS) Using a Monochromated TEM/STEM

A 200 keV FEI TF20 XT monochromated (scanning) transmission electron microscope funded by NASA's SRLIDAP program is undergoing installation at Lawrence Livermore National Laboratory. Instrument specifications in STEM mode are Cs =1.0 mm, Cc =1.2 mm, image resolution =0.18 nm, and in TEM mode Cs =1.3 mm, Cc =1.3 mm, information limit =0.14 nm. Key features of the instrument are a voltage-stabilized high tension (HT) supply, a monochromator, a high-resolution electron energy-loss spectrometer/energy filter, a high-resolution annular darkfield detector, and a solid-state x-ray energy-dispersive spectrometer. The high-tension tank contains additional sections for 60Hz and high frequency filtering, resulting in an operating voltage of 200 kV plus or minus 0.005V, a greater than 10-fold improvement over earlier systems. The monochromator is a single Wien filter design. The energy filter is a Gatan model 866 Tridiem-ERS high resolution GIF spec d for less than or equal to 0.15 eV energy resolution with 29 pA of current in a 2 nm diameter probe. 0.13 eV has already been achieved during early installation. The x-ray detector (EDAX/Genesis 4000) has a take-off angle of 20 degrees, an active area of 30 square millimeters, and a solid angle of 0.3 steradians. The higher solid angle is possible because the objective pole-piece allows the detector to be positioned as close as 9.47 mm from the specimen. The voltage-stabilized HT supply, monochromator and GIF enable high-resolution electron energy-loss spectroscopy (HREELS) with energy resolution comparable to synchrotron XANES, but with approximately 100X better spatial resolution. The region between 0 and 100 eV is called the low-loss or valence electron energy-loss spectroscopy (VEELS) region where features due to collective plasma oscillations and single electron transitions of valence electrons are observed. Most of the low-loss VEELS features we are detecting are being observed for the first time in IDPs. A major focus of our research is to understand the origin and significance of these features and how they might be exploited to gain insight about IDPs and other meteoritic materials.

Sai, Z. R.↗
NASA NTRS2005

Ultrasonic Micro-Blades for the Rapid Extraction of Impact Tracks from Aerogel

The science return of NASA's Stardust Mission with its valuable cargo of cometary debris hinges on the ability to efficiently extract particles from silica aerogel collectors. The current method for extracting cosmic dust impact tracks is a mature procedure involving sequential perforation of the aerogel with glass needles on computer controlled micromanipulators. This method is highly successful at removing well-defined aerogel fragments of reasonable optical clarity while causing minimal damage to the surrounding aerogel collector tile. Such a system will be adopted by the JSC Astromaterials Curation Facility in anticipation of Stardust s arrival in early 2006. In addition to Stardust, aerogel is a possible collector for future sample return missions and is used for capture of hypervelocity ejecta in high power laser experiments of interest to LLNL. Researchers will be eager to obtain Stardust samples for study as quickly as possible, and rapid extraction tools requiring little construction, training, or investment would be an attractive asset. To this end, we have experimented with micro-blades for the Stardust impact track extraction process. Our ultimate goal is a rapid extraction system in a clean electron beam environment, such as an SEM or dual-beam FIB, for in situ sample preparation, mounting and analysis.

Ishii, H. A.↗
NASA NTRS2005

Nitrogen Isotopic Anomalies in a Hydrous Interplanetary Dust Particle

Interplanetary dust particles (IDPs) collected in the stratosphere are the fine-grained end member (5 - 50 microns in size) of the meteoritic material available for investigation in the laboratory. IDPs are derived from either cometary or asteroidal sources. Some IDPs contain cosmically primitive materials with isotopic signatures reflecting presolar origins. Recent detailed studies using the NanoSIMS have shown there is a wide variation of isotopic signatures within individual IDPs; grains with a presolar signature have been observed surrounded by material with a solar isotopic composition. The majority of IDPs studied have been anhydrous. We report here results from integrated NanoSIMS/FIB/TEM/Synchrotron IR studies of a hydrous IDP, focused on understanding the correlations between the isotopic, mineralogical and chemical compositions of IDPs.

Smith, J. B.↗
NASA NTRS2004

Focused Ion Beam Recovery and Analysis of Interplanetary Dust Particles (IDPs) and Stardust Analogues

Meteoritics research is a major beneficiary of recent developments in analytical instrumentation [1,2]. Integrated studies in which multiple analytical techniques are applied to the same specimen are providing new insight about the nature of IDPs [1]. Such studies are dependent on the ability to prepare specimens that can be analyzed in multiple instruments. Focused ion beam (FIB) microscopy has revolutionized specimen preparation in materials science [3]. Although FIB has successfully been used for a few IDP and meteorite studies [1,4-6], it has yet to be widely utilized in meteoritics. We are using FIB for integrated TEM/NanoSIMS/synchrotron infrared (IR) studies [1].

Graham, G. A.↗
NASA NTRS2003

The Petrography of Meteoritic Nano-Diamonds

At least some meteoritic nanodiamonds are likely of presolar origin because of their association with anomalous Xe-HL and Te isotopic components indicative of a supernova (SN) origin. But the abundance of Xe is such that only approx. 1 in 10(exp 6) nano-diamonds contains a Xe atom, and the bulk C-13/C-12 composition of nano-diamond acid residues is chondritic (solar). Therefore, it is possible that a significant fraction of meteoritic nano-diamonds formed within the solar nebula. Nano-diamonds have recently been detected for the first time within the accretion discs of young stars by the Infrared Space Observatory (ISO). No comparable evidence of nanodiamonds in the interstellar medium has yet been found. We have identified nano-diamonds in acid etched thin-sections of meteorites, polar micrometeorites, and interplanetary dust particles (IDPs) with the goal of determining their distribution as a function of heliocentric distance. (It is assumed the meteorites and the polar micrometeorites are from asteroids at 2-4 AU and at least some of the IDPs are from comets at >50AU). We found that nano-diamonds are heterogeneously distributed throughout carbon-rich meteoritic materials (we identified them in some IDPs and not in others), and that their abundance may actually decrease with heliocentric distance, consistent with the hypothesis that some of them formed within the inner solar system and not in a presolar (SN) environment. In order to gain further insight about the origins of meteoritic nano-diamonds we are currently investigating their distribution in unetched thin-sections. We have examined a chondritic cluster IDP (U220GCA), fragments of the Tagish Lake (CM1) meteorite, and a SN graphite spherule (KE3d8) isolated from the Murchison (CM) meteorite. We selected U220GCA because its nano-diamond abundance (in acid etched thin-sections) appears to be as much as approx. 10X higher than in Murchison matrix, Tagish Lake because it has a higher reported nano-diamond abundance than other carbonaceous chondrites (approx. 3650-4330 ppm), and KE3d8 because it is a carbon spherule with an isotopic composition suggesting that it is a bone fide presolar SN grain.

Dai, Z. R.↗
NASA NTRS2003

An Extraction and Curation Technique for Particles Captured in Aerogel Collectors

In 2006, the STARDUST mission will return samples of cometary, interplanetary and interstellar dust captured in aerogel collectors. The curation strategy (or strategies) for STARDUST remain to be defined. Here we present a technique for curation of particles captured in aerogel which will simultaneously preserve the entire particle track, and could allow for the elemental in situ analysis of both the particle residue at the terminus of the track and any particle residues located along the track. This builds on work that we have previously reported elsewhere.

Westphal, A. J.↗
NASA NTRS1999

An infrared spectral match between GEMS and interstellar grains

Infrared spectral properties of silicate grains in interplanetary dust particles (IDPs) were compared with those of astronomical silicates. The approximately 10-micrometer silicon-oxygen stretch bands of IDPs containing enstatite (MgSiO3), forsterite (Mg2SiO4), and glass with embedded metal and sulfides (GEMS) exhibit fine structure and bandwidths similar to those of solar system comets and some pre-main sequence Herbig Ae/Be stars. Some GEMS exhibit a broad, featureless silicon-oxygen stretch band similar to those observed in interstellar molecular clouds and young stellar objects. These GEMS provide a spectral match to astronomical "amorphous" silicates, one of the fundamental building blocks from which the solar system is presumed to have formed.

NASA Discipline Exobiology↗