Search

NASA's Stardust mission utilized a sample collector composed of aerogel and aluminum foil to return cometary and interstellar particles to Earth. Analysis of the aluminum foil begins with locating craters produced by hypervelocity impacts of cometary and interstellar dust. Interstellar dust craters are typically less than one micrometer in size and are sparsely distributed, making them difficult to find. In this paper, we describe a convolutional neural network based on the VGG16 architecture that achieves high specificity and sensitivity in locating impact craters in the Stardust interstellar collector foils. We evaluate its implications for current and future analyses of Stardust samples. [ABSTRACT FROM AUTHOR]

Aluminum foils from the Stardust cometary dust collector contain impact craters formed during the spacecraft's encounter with comet 81P/Wild 2 and retain residues that are among the few unambiguously cometary samples available for laboratory study. Our study investigates four micron‐scale (1.8–5.2 μm) and six submicron (220–380 nm) diameter craters to better characterize the fine (<1 μm) component of comet Wild 2. We perform initial crater identification with scanning electron microscopy, prepare the samples for further analysis with a focused ion beam, and analyze the cross sections of the impact craters with transmission electron microscopy (TEM). All of the craters are dominated by combinations of silicate and iron sulfide residues. Two micron‐scale craters had subregions that are consistent with spinel and taenite impactors, indicating that the micron‐scale craters have a refractory component. Four submicron craters contained amorphous residue layers composed of silicate and sulfide impactors. The lack of refractory materials in the submicron craters suggests that refractory material abundances may differentiate Wild 2 dust on the scale of several hundred nanometers from larger particles on the scale of a micron. The submicron craters are enriched in moderately volatile elements (S, Zn) when normalized to Si and CI chondrite abundances, suggesting that, if these craters are representative of the Wild 2 fine component, the Wild 2 fines were not formed by high‐temperature condensation. This distinguishes the comet's fine component from the large terminal particles in Stardust aerogel tracks which mostly formed in high‐temperature events. [ABSTRACT FROM AUTHOR]

Abstract: Over the last decade, silica aerogel tracks and aluminum foil craters on the Stardust collector have been studied extensively to determine the nature of captured cometary dust grains. Analysis of particles captured in aerogel has been developed to a fine art, aided by sophisticated preparation techniques, and yielding revolutionary knowledge of comet dust mineralogy. The Stardust foil craters can be interpreted in terms of impacting particle size and structure, but almost all studies of composition for their contents have relied on in situ analysis techniques or relatively destructive extraction of materials. This has limited their examination and interpretation. However, numerous experimental hypervelocity impact studies under Stardust‐Wild 2 encounter conditions have shown that abundant dust components are preserved in foil craters of all sizes. Using some of these analogue materials, we have previously shown that modern, nondestructive scanning electron microscope imaging and X‐ray microanalysis techniques can document distribution of dust remnants both quickly and thoroughly within foil craters prior to any preparation. Here we present findings from our efforts to quantify the amount of residue and demonstrate a simple method of crater shape modification which can bring material into positions where it is much more accessible for in situ analysis, or safe removal of small subsamples. We report that approximately 50% of silicate‐dominated impactors were retained as impact crater residue; however, <3% of organic impactors remained in the craters after impact. [ABSTRACT FROM AUTHOR]

We compare the solar wind ( SW) He, Ne, and Ar compositions collected during the Apollo Solar Wind Composition ( SWC) experiments (1969-1972; Al- & Pt-foils) and the Genesis mission (2002-2004; so-called DOS targets considered here). While published SW 20Ne/22Ne and 36Ar/38Ar ratios of both data sets agree, differences exist in the 4He/3He, 4He/20Ne, and 20Ne/36Ar ratios. However, 20Ne/36Ar ratios from Apollo-16 Pt-foils, exclusively adopted as SW values by the SWC team, are consistent with the Genesis results. We investigate if the differences indicate a variability of the SW over the course of about 30 yr, or systematic biases of the two data sets, which were collected in different environments and measured several decades apart in different laboratories (University of Bern; ETH Zurich). New measurements of Apollo-15 SWC aluminum foils in Zurich generally agree with the original measurements performed in Bern. Zurich samples show slightly lower 4He concentrations suggesting a few percent of diffusive loss of 4He during storage of the foils. A 3% difference between the He isotopic ratios measured in Bern and in Zurich possibly represents an analytical bias between the laboratories. The low SW 4He/20Ne and 20Ne/36Ar ratios in Apollo-15 Al-foils compared to Genesis data are consistent with a mixture of Genesis-like SW and noble gases from small amounts of lunar dust. Our data suggest that the mean SW He, Ne, and Ar isotopic and elemental compositions have not significantly changed between the overall Apollo and Genesis mission collection periods. [ABSTRACT FROM AUTHOR]

Introduction: On April 27, 2022 at 8:03 a.m. CDT (1303 GMT) multiple eyewitnesses from Arkansas, Louisiana and Mississippi reported a bolide and sonic booms (AMS Event 2022-2591)[1]. NASA-Marshall Space Flight Center's Meteoroid Environment Office Lead Scientist Bill Cooke shared the event at 4:51p.m. on the NASA Meteor Watch Facebook Page. Cooke reported that the bolide was produced by a 40 kg object of approximately 0.3 meters in diameter, moving at approximately 56,327 kph. Observations show that the object broke apart roughly 34 miles above the ground. Late on the 28th, citizen scientist Eric Rasmussen contacted NASA- Johnson Space Center (NASA-JSC) scientist Marc Fries suggesting that there were very strong Doppler radar reflections near the time of the reported fireball. Fries posted his Doppler radar and dark flight model results on 29 April on the NASA-ARES website. On April 30, 2022 at 2:44 p.m., the first two meteorites were recovered 16.9km east of Natchez, MS on the shoulder (31°33'06.1"N 91°11'36.6"W) and median (31°33'08.9"N 91°11'25.9"W) of U.S. Hwy 84E. When officially confirmed, this meteorite will represent the fifth officially approved meteorite from Mississippi. It is the first recovered in MS since Tupelo (EL6 find) in 2012, and also the first fall in MS in 100 years, since the Baldwyn L6 chondrite fell in 1922. Strewnfield Model: Doppler radar returns. Weather radars (Fig. 1 top) in the NOAA NEXRAD network detected signatures of falling meteorites in eleven radar sweeps by four separate radars [2]. The first detection occurred 73 s after bolide terminus at an altitude of 11,257 m above sea level (ASL). The last detection occurred 358 s after terminus with detections occurring between 2,115 and 13,704 m ASL. Early strewnfield map. A strewnfield map was calculated using the Jörmungandr dark flight model (Fig. 1 top). Average mass in each radar sweep was estimated using change in altitude and time from the terminus. Masses range from 1.3 to 251g, with the caveat that larger masses commonly evade radar detection. The calculated strewn field estimates where meteorites of a given mass may lie, with the simpifying assumption that all meteorites originate from a common point at the terminus. The estimated strewn field and calculated landing sites for the radar signatures agree very well (Fig. 1 bottom). Preliminary Type Classification: The two stones, N1 (41.31g) (Fig. 2) and N2 (37.64g) (Fig. 3), were collected 30 April prior to 1 May rain, cleanly, using baked-out aluminium foil and bagged in Teflon for transport. The samples were weighed two days later at NASA-JSC, photographed and then measured for their magnetic susceptibility. The Log X value for N1 is 5.06 and for N2 is 5.08, suggesting that the stones fall in the range of values expected for L chondrites [3]. N2 is a broken stone revealing a matrix with clasts ranging from millimeters to centimeters in size. Clasts are poorly defined at margins, irregularly shaped and include a variety of textures suggesting that the meteorite represents a polymict breccia. Chondrules, when visible, are better preserved within clasts. The surrounding darker mottled matrix shows a wide range of millimeter-sized lighter colored irregularly shaped grains and chondrule fragments. Imagery of other stone fragments are also of mixed color and texture. XCT will be employed to characterize the range and degree of brecciation. Microprobe analyses are underway to confirm chemistry and petrology for a complete classification and subsequent submission to the Meteoritical Society Nomenclature committee. [ABSTRACT FROM AUTHOR]

Introduction: The Winchcombe meteorite fell on 28th February 2021. At 21.54, the fireball was witnessed across the UK and recorded by 16 stations operated by the six meteor camera networks of the UK Fireball Alliance (UKFAll), allowing for calculation of its strewn field and main belt source [1]. The event was widely observed by eyewitnesses and soon attracted considerable media attention. The first carbonaceous chondrite recovered in the UK, Winchcombe joins a short list of only 17 British falls for which material is known [2]. Recovery: The recovered material was initially examined by team members in the field, conducting home visits to potential finds and systematically searching the local area. Permission for minimally destructive analysis on the material was sought from all landowners. The largest mass landed on the driveway and lawn of a fortunate family who collected much of the material with rubber gloves and a stainless steel knife, storing it in cleaned supermarket cottage cheese/cream pots, and polyethylene sandwich bags. Some material was collected as soon as 12 hours after the fall. Additional material was collected at this site by the recovery team who, using nitrile gloves, tweezers, paintbrushes and a toothbrush, transferred the material directly into conservation-grade storage materials (aluminium foil, glass vials and polyethylene sample bags) provided by the Natural History Museum (NHM) meteorite collection [3]. The largest single stone was found in a field of Rushbury House Farm, during a search by the recovery team on 6th March, and collected using nitrile gloves and geological sample bags. The quick response of UKFAll and associated media campaign was critical in raising awareness of how to identify a meteorite and how to handle specimens appropriately, resulting in many enquiries from the public. In total, approximately 602 g have been recovered from sites, such as driveways, gardens, fields and a solar farm, across an 8.5 km strewnfield. Immediate curation: The first samples were received and placed into a desiccator at the NHM on the 4th. In the following fortnight, 709 g of material (including mud containing small meteorite fragments), from six sites, had been visually examined and documented under the NHM object entry process to enable investigation prior to official acquistion. Samples were quickly curated, and are stored in glass vials with polyethylene lids, with fragments > ~100 mg individually weighed. Twenty-three of the largest fragments have been encased in high purity Al foil, placed in acid-free cardboard trays and stored in heat-sealed Escal enclosures with Mitsubishi RPK system oxygen scavengers. The largest piece is stored in an individual dessicator and is currently on display in the Vault gallery in the NHM. Two stones (BM.2022,M1-85 and BM.2022,M1-86) were taken directly from the field to the Open University to facilitate swift oxygen isotope measurements and are stored in a nitrogen cabinet in a class 100 cleanroom, under long-term loan from the NHM. Additionally, some material has been returned to the landowners for donation to local museums and thus not accessioned into the NHM collection. Tracking subsamples: Thanks to the generosity of the local community, a total of 518.3 g have now been donated into the NHM collection. Specimens have been designated according to their find location and collection date. The subsamples are tracked with sequential numbers, similar to the NASA curatorial numbering system. The subsample relationship of any given fragment can be provided upon request to the curator. Thus far, 47 loans, a total of approximately 80 g (including non-destructive analyes) and 20 polished sections have been made available to the Winchcombe consortium, covering analyses including bulk properties, mineralogy and petrology of the different lithologies, fusion crust, organics, magnetics, reflectance spectroscopy, and terrestrial alteration, as well as C, N, O, Ti, Cr, H, Ne isotopic systems [1, 3-16]. Long term curation: A nitrogen glove box (MBraun Labstar) with built-in microscope is on order and will be used for the permanent storage of the Winchcombe meteorite, funded by the Science and Technology Facilities Council (STFC), UK. Samples of the mud, lawn, and the original collecting vessels are all available as witness material. We welcome requests for scientific loans once the initial consortium work is published in a special issue of Meteoritics and Planetary Science. [ABSTRACT FROM AUTHOR]

The Stardust Interstellar Preliminary Examination team analyzed thirteen Al foils from the NASA Stardust interstellar collector tray in order to locate candidate interstellar dust (ISD) grain impacts. Scanning electron microscope ( SEM) images reveal that the foils possess abundant impact crater and crater-like features. Elemental analyses of the crater features, with Auger electron spectroscopy, SEM-based energy dispersive X-ray ( EDX) spectroscopy, and scanning transmission electron microscope-based EDX spectroscopy, demonstrate that the majority are either the result of impacting debris fragments from the spacecraft solar panels, or intrinsic defects in the foil. The elemental analyses also reveal that four craters contain residues of a definite extraterrestrial origin, either as interplanetary dust particles or ISD particles. These four craters are designated level 2 interstellar candidates, based on the crater shapes indicative of hypervelocity impacts and the residue compositions inconsistent with spacecraft debris. [ABSTRACT FROM AUTHOR]

- We have shown in laboratory experiment that hypervelocity impacts on a solar cell produce ejecta that can be captured on aluminum (Al 1100) foil or in low density (33 kg m−3) aerogel. The origin of the secondary impacts can be determined by either analysis of the residue in the craters in the foils (which preserve an elemental signature of the solar cell components) or by their pointing direction for tracks in the aerogel (which we show align with the impact direction to ± 0.4°). This experimental evidence explains the observations of the NASA Stardust mission which has reported that the majority of tracks in the aerogel collector used to collect interstellar dust actually point at the spacecraft's solar panels. From our results, we suggest that it should also be possible to recognize secondary ejecta craters in the Stardust mission aluminum foils, also used as dust sampling devices during the mission. [ABSTRACT FROM AUTHOR]

- We present NanoSIMS four-isotope S analyses of 24 comet Wild 2 dust impact residues in craters on aluminum foil C2037N returned by NASA's Stardust mission. Except for one sample, all impact residues have normal S isotopic compositions within 2σ uncertainties of at least two S isotope ratios. This implies that most S-rich Wild 2 dust impactors formed in the solar system. Instrumental isotope fractionation due to sample topography is the main contribution to our analytical uncertainty. One impact crater residue shows small anomalies of δ33S = −57 ± 17‰, and δ34S = −41 ± 17‰ (1σ uncertainties). Although this could be simply a statistical outlier or the fingerprint of a chemical isotope fractionation it is also possible that the observed anomaly results from the mixture of a cometary FeS particle with a small (150 nm diam.) presolar FeS supernova grain. This would translate into a presolar sulfide abundance of approximately 200 ppm. [ABSTRACT FROM AUTHOR]

- The Al foils lining the aerogel tiles of the Stardust interstellar tray represent approximately 13% of the total collecting area, about 15,300 mm2. Although the flux is poorly constrained, fewer than 100 impacts are expected in all the Al foils on the collector, and most of these are likely to be less than 1 μm in diameter. Secondary electron (SE) images of the foils at a resolution of approximately 50 nm per pixel are being collected during the Stardust Interstellar Preliminary Examination, resulting in more than two million images that will eventually need to be searched for impact craters. The unknown and complicated nature of 3-dimensional interstellar tracks in aerogel necessitated the use of a massively distributed human search to locate only a few interstellar tracks. The 2-dimensional nature of the SE images makes the problem of searching for craters tractable for algorithmic approaches. Using templates of craters from cometary impacts into Stardust foils, we present a computer algorithm for the identification of impact craters in the Stardust interstellar foils using normalized cross-correlation and template matching. We address the speed, sensitivity, and false-positive rate of the algorithm. The search algorithm can be adapted for use in other applications. The program is freely available for download at . [ABSTRACT FROM AUTHOR]

- The fluence of dust particles <10 μm in diameter was recorded by impacts on aluminum foil of the NASA Stardust spacecraft during a close flyby of comet 81P/Wild 2 in 2004. Initial interpretation of craters for impactor particle dimensions and mass was based upon laboratory experimental simulations using projectiles less than >10 μm in diameter and the resulting linear relationship of projectile to crater diameter was extrapolated to smaller sizes. We now describe a new experimental calibration program firing very small monodisperse silica projectiles (470 nm-10 μm) at approximately 6 km s−1. The results show an unexpected departure from linear relationship between 1 and 10 μm. We collated crater measurement data and, where applicable, impactor residue data for 596 craters gathered during the postmission preliminary examination phase. Using the new calibration, we recalculate the size of the particle responsible for each crater and hence reinterpret the cometary dust size distribution. We find a greater flux of small particles than previously reported. From crater morphology and residue composition of a subset of craters, the internal structure and dimensions of the fine dust particles are inferred and a 'maximum-size' distribution for the subgrains composing aggregate particles is obtained. The size distribution of the small particles derived directly from the measured craters peaks at approximately 175 nm, but if this is corrected to allow for aggregate grains, the peak in subgrain sizes is at <100 nm. [ABSTRACT FROM AUTHOR]