Correlated microanalysis of cometary organic grains returned by Stardust

Abstract– Carbonaceous matter in Stardust samples returned from comet 81P/Wild 2 is observed to contain a wide variety of organic functional chemistry. However, some of this chemical variety may be due to contamination or alteration during particle capture in aerogel. We investigated six carbonaceous Stardust samples that had been previously analyzed and six new samples from Stardust Track 80 using correlated transmission electron microscopy (TEM), X‐ray absorption near‐edge structure spectroscopy (XANES), and secondary ion mass spectroscopy (SIMS). TEM revealed that samples from Track 35 containing abundant aliphatic XANES signatures were predominantly composed of cometary organic matter infilling densified silica aerogel. Aliphatic organic matter from Track 16 was also observed to be soluble in the epoxy embedding medium. The nitrogen‐rich samples in this study (from Track 22 and Track 80) both contained metal oxide nanoparticles, and are likely contaminants. Only two types of cometary organic matter appear to be relatively unaltered during particle capture. These are (1) polyaromatic carbonyl‐containing organic matter, similar to that observed in insoluble organic matter (IOM) from primitive meteorites, interplanetary dust particles (IDPs), and in other carbonaceous Stardust samples, and (2) highly aromatic refractory organic matter, which primarily constitutes nanoglobule‐like features. Anomalous isotopic compositions in some of these samples also confirm their cometary heritage. There also appears to be a significant labile aliphatic component of Wild 2 organic matter, but this material could not be clearly distinguished from carbonaceous contaminants known to be present in the Stardust aerogel collector.     

Investigation of iron sulfide impact crater residues: A combined analysis by scanning and transmission electron microscopy

Abstract– Samples returned from comet 81P/Wild 2 by the Stardust mission provided an unequaled opportunity to compare previously available extraterrestrial samples against those from a known comet. Iron sulfides are a major constituent of cometary grains commonly identified within cometary interplanetary dust particles (IDPs) and Wild 2 samples. Chemical analyses indicate Wild 2 sulfides are fundamentally different from those in IDPs. However, as Wild 2 dust was collected via impact into capture media at approximately 6.1 km s^?1, it is unclear whether this is due to variation in preaccretional/parent body processes experienced by these materials or due to heating and alteration during collection. We investigated alteration in pyrrhotite and pentlandite impacted into Stardust flight spare Al foils under encounter conditions by comparing scanning and transmission electron microscope (SEM, TEM) analyses of preimpact and postimpact samples and calculating estimates of various impact parameters. SEM is the primary method of analysis during initial in situ examination of Stardust foils, and therefore, we also sought to evaluate the data obtained by SEM using insights provided by TEM. We find iron sulfides experience heating, melting, separation, and loss of S, and mixing with molten Al. These results are consistent with estimated peak pressures and temperatures experienced (approximately 85 GPa, approximately 2600 K) and relative melting temperatures. Unambiguous identification of preserved iron sulfides may be possible by TEM through the location of Al‐free regions. In most cases, the Ni:Fe ratio is preserved in both SEM and TEM analyses and may therefore also be used to predict original chemistry and estimate mineralogy.     

Complex aromatic hydrocarbons in Stardust samples collected from comet 81P/Wild 2

Abstract– The successful return of the Stardust spacecraft provides a unique opportunity to investigate the nature and distribution of organic matter in cometary dust particles collected from comet 81P/Wild 2. Analysis of individual cometary impact tracks in silica aerogel using the technique of two‐step laser mass spectrometry demonstrates the presence of complex aromatic organic matter. While concerns remain as to the organic purity of the aerogel collection medium and the thermal effects associated with hypervelocity capture, the majority of the observed organic species appear indigenous to the impacting particles and are hence of cometary origin. While the aromatic fraction of the total organic matter present is believed to be small, it is notable in that it appears to be N rich. Spectral analysis in combination with instrumental detection sensitivies suggest that N is incorporated predominantly in the form of aromatic nitriles (R–C≡N). While organic species in the Stardust samples do share some similarities with those present in the matrices of carbonaceous chondrites, the closest match is found with stratospherically collected interplanetary dust particles. These findings are consistent with the notion that a fraction of interplanetary dust is of cometary origin. The presence of complex organic N containing species in comets has astrobiological implications as comets are likely to have contributed to the prebiotic chemical inventory of both the Earth and Mars.     

Thermochemical stability of low‐iron,manganese‐enriched olivine in astrophysical environments

Abstract– Low‐iron, manganese‐enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission from comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix. LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to those of a very reducing vapor of solar composition. The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without re‐equilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.     

FIB‐TEM analysis of cometary material in 10 Stardust foil craters

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.     

Raman characterization of carbonaceous matter in CONCORDIA Antarctic micrometeorites

Abstract– We report a multi‐wavelength Raman spectroscopy study of carbonaceous matter in 38 Antarctic micrometeorites (AMMs) from the 2006 CONCORDIA collection. The particles were selected as a function of their degree of thermal alteration developed during the deceleration in the atmosphere. These samples range from unmelted (fine‐grained—Fg; ultracarbonaceous—UCAMMs) to partially melted AMMs (scorias—Sc) and completely melted particles (cosmic spherules—CS). More than half of the analyzed AMMs contain a substantial amount of polyaromatic carbonaceous matter with a high degree of disorder. The proportion of particles where carbon is not detected increase from the Fg to the Fg‐Sc and to the Sc‐AMMs, and no carbon is detected in CS. In addition, the spectral characteristics of the G and D bands of the carbonaceous matter in Sc‐AMMs plot apart from the trend formed by the data from Fg‐AMMs and UCAMMs. These results suggest that oxidation processes occurred during the deceleration of the particles in the atmosphere. In Fg‐AMMs and UCAMMs, the spectral characteristics of the G and D bands reveal the high degree of disorder of the carbonaceous matter, precluding a long duration thermal metamorphism on the parent body and suggesting that AMMs have a connection with C1–C2 chondrites. The Raman parameters of the deuterium‐rich carbonaceous matter of UCAMMs do not differ from that of Fg‐AMMs. Using a 244 nm excitation, we detected the cyanide (–CN) functional group for the first time in a UCAMM, reinforcing the likely cometary origin of this type of micrometeorites.     

Refractory materials in comet samples

Transmission electron microscope examination of more than 250 fragments, >1 μm from comet Wild 2 and a giant cluster interplanetary dust particle (GCP) of probable cometary origin has revealed four new calcium‐aluminum‐rich inclusions (CAIs), an amoeboid olivine aggregate (AOA), and an additional AOA or Al‐rich chondrule (ARC) object. All of the CAIs have concentric mineral structures and are composed of spinel + anorthite cores surrounded by Al,Ti clinopyroxenes and are similar to two previous CAIs discovered in Wild 2. All of the cometary refractory objects are of moderate refractory character. The mineral assemblages, textures, and bulk compositions of the comet CAIs are similar to nodules in fine‐grained, spinel‐rich inclusions (FGIs) found in primitive chondrites and like the nodules may be nebular condensates that were altered via solid–gas reactions in the solar nebula. Oxygen isotopes collected on one Wild 2 CAI also match FGIs. The lack of the most refractory inclusions in the comet samples may reflect the higher abundances of small moderately refractory CAI nodules that were produced in the nebula and the small sample sizes collected. In the comet samples, approximately 2–3% of all fragments larger than 1 μm, by number, are CAIs and nearly 50% of all bulbous Stardust tracks contain at least one CAI. We estimate that ~0.5 volume % of Wild 2 material and ~1 volume % of GCP is in the form of CAIs. ARCs and AOAs account for <1% of the Wild 2 and GCP grains by number.     

Organic compound alteration during hypervelocity collection of carbonaceous materials in aerogel

Abstract— The NASA Stardust mission brought to Earth micron‐size particles from the coma of comet 81P/Wild 2 using aerogel, a porous silica material, as the capture medium. A major challenge in understanding the organic inventory of the returned comet dust is identifying, unambiguously, which organic molecules are indigenous to the cometary particles, which are produced from carbon contamination in the Stardust aerogel, and which are cometary organics that have been modified by heating during the particle capture process. Here it is shown that 1) alteration of cometary organic molecules along impact tracks in aerogel is highly dependent on the original particle morphology, and 2) organic molecules on test‐shot terminal particles are mostly preserved. These conclusions are based on two‐step laser mass spectrometry (L²MS) examinations of test shots with organic‐laden particles (both tracks in aerogel and the terminal particles themselves).     

Short-period Jupiter family comets after Stardust

The NASA Stardust mission has provided for laboratory study an extensive data set of cometary dust of known provenance (from comet 81P/Wild 2) yielding detailed insights into the composition of the comet. Combined with the results of data from other missions to short-period Jupiter family comets (JFC), this has greatly deepened the understanding of such objects. If depressions on the surface of comet 81P/Wild 2 are all taken as evidence of impact cratering, their number suggests a long occupancy in the outer region of the Solar System. The dust from comet 81P/Wild 2 has been shown to be heavily deficient in pre-Solar grains and rich in materials formed at high temperatures in the inner Solar System. Although it is too early to know if this is typical of JFC, it does argue for rapid and thorough mixing of materials in the disk on timescales related to comet formation, and may also suggest outward migration of small icy bodies after their formation. Thus, instead of providing mainly new knowledge of the pre-Solar materials expected to be rich in comets, Stardust and comet 81P/Wild 2 have instead focussed attention on large-scale transport processes during the critical period when cometary parent bodies were forming in the early Solar System.     

Oxygen isotope composition of meteoritic ablation debris from the Transantarctic Mountains: Constraining the parent body and implications for the impact scenario

Abstract– Microscopic meteoritic ablation spheres recently found on top of the Victoria Land in Transantarctic Mountains, and in the L2 Dome C and DF2691 Dome Fuji ice core layers document a major impact of a 10⁸ kg (or larger) cosmic body in the Antarctic region about 480 kyr ago. Although of broadly chondritic composition, the exact nature of the impactor is unknown, and whether the impactor struck the Antarctic ice sheet or exploded in the atmosphere is a matter of debate. Based on oxygen isotope analyses of ablation spheres from the Transantarctic Mountains by means of IR‐laser fluorination coupled with mass spectrometry, we suggest that they represent the debris of an atmospheric airburst of a primitive asteroid of CV, CO, or CK composition, or a comet with composition similar to the short‐period comet 81P/Wild 2.     

Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues

Abstract— The known encounter velocity (6.1 kms^?1) and particle incidence angle (perpendicular) between the Stardust spacecraft and the dust emanating from the nucleus of comet Wild‐2 fall within a range that allows simulation in laboratory light‐gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre‐existing major‐ and trace‐element composition of the foil, geometrical issues for energy dispersive X‐ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.     

Coordinated mineralogical and isotopic analyses of a cosmic symplectite discovered in a comet 81P/Wild 2 sample

We have discovered in a Stardust mission terminal particle a unique mineralogical assemblage of symplectically intergrown pentlandite ((Fe,Ni)₉S₈) and nanocrystalline maghemite (γ‐Fe₂O₃). Mineralogically similar cosmic symplectites (COS) have only been found in the primitive carbonaceous chondrite Acfer 094 and are believed to have formed by aqueous alteration. The O and S isotopic compositions of the Wild 2 COS are indistinguishable from terrestrial values. The metal and sulfide precursors were thus oxidized by an isotopically equilibrated aqueous reservoir either inside the snow line, in the Wild 2 comet, or in a larger Kuiper Belt object. Close association of the Stardust COS with a Kool mineral assemblage (kosmochloric Ca‐rich pyroxene, FeO‐rich olivine, and albite) that likely originated in the solar nebula suggests the COS precursors also had a nebular origin and were transported from the inner solar system to the comet‐forming region after they were altered.     

Aerogel tracks made by impacts of glycine: Implications for formation of bulbous tracks in aerogel and the Stardust mission

Abstract– Impacts of small particles of soda‐lime glass and glycine onto low density aerogel are reported. The aerogel had a quality similar to the flight aerogels carried by the NASA Stardust mission that collected cometary dust during a flyby of comet 81P/Wild 2 in 2004. The types of track formed in the aerogel by the impacts of the soda‐lime glass and glycine are shown to be different, both qualitatively and quantitatively. For example, the soda‐lime glass tracks have a carrot‐like appearance and are relatively long and slender (width to length ratio <0.11), whereas the glycine tracks consist of bulbous cavities (width to length ratio >0.26). In consequence, the glycine particles would be underestimated in diameter by a factor of 1.7–3.2, if the glycine tracks were analyzed using the soda‐lime glass calibration and density. This implies that a single calibration for impacting particle size based on track properties, as previously used by Stardust to obtain cometary dust particle size, is inappropriate.     

Laboratory simulation of impacts on aluminum foils of the Stardust spacecraft: Calibration of dust particle size from comet Wild‐2

Abstract— Metallic aluminum alloy foils exposed on the forward, comet‐facing surface of the aerogel tray on the Stardust spacecraft are likely to have been impacted by the same cometary particle population as the dedicated impact sensors and the aerogel collector. The ability of soft aluminum alloy to record hypervelocity impacts as bowl‐shaped craters offers an opportunistic substrate for recognition of impacts by particles of a potentially wide size range. In contrast to impact surveys conducted on samples from low Earth orbit, the simple encounter geometry for Stardust and Wild‐2, with a known and constant spacecraft‐particle relative velocity and effective surface‐perpendicular impact trajectories, permits closely comparable simulation in laboratory experiments. For a detailed calibration program, we have selected a suite of spherical glass projectiles of uniform density and hardness characteristics, with well‐documented particle size range from 10 μm to nearly 100 μm. Light gas gun buckshot firings of these particles at approximately 6 km s^?1 onto samples of the same foil as employed on Stardust have yielded large numbers of craters. Scanning electron microscopy of both projectiles and impact features has allowed construction of a calibration plot, showing a linear relationship between impacting particle size and impact crater diameter. The close match between our experimental conditions and the Stardust mission encounter parameters should provide another opportunity to measure particle size distributions and fluxes close to the nucleus of Wild‐2, independent of the active impact detector instruments aboard the Stardust spacecraft.     

Exposure and analysis of microparticles embedded in silica aerogel keystones using NF3‐mediated electron beam–induced etching and energy‐dispersive X‐ray spectroscopy

In 2006, NASA's Stardust spacecraft delivered to Earth dust particles collected from the coma of comet 81P/Wild 2, with the goal of furthering the understanding of solar system formation. Stardust cometary samples were collected in a low‐density, nanoporous silica aerogel making their study technically challenging. This article demonstrates the identification, exposure, and elemental composition analysis of particles analogous to those collected by NASA's Stardust mission using in‐situ SEM techniques. Backscattered electron imaging is shown by experimental observation and Monte Carlo simulation to be suitable for locating particles of a range of sizes relevant to Stardust (down to submicron diameters) embedded within silica aerogel. Selective removal of the silica aerogel encapsulating an embedded particle is performed by cryogenic NF₃‐mediated electron beam–induced etching. The porous, low‐density nature of the aerogel results in an enhanced etch rate compared with solid material, making it an effective, nonmechanical method for the exposure of particles. After exposure, elemental composition of the particle was analyzed by energy‐dispersive X‐ray spectroscopy using a high spectral resolution microcalorimeter. Signals from fluorine contamination are shown to correspond to nonremoved silica aerogel and only in residual concentrations.     

Overview of the rocky component of Wild 2 comet samples: Insight into the early solar system,relationship with meteoritic materials and the differences between comets and asteroids

Abstract– The solid 2–10 μm samples of comet Wild 2 provide a limited but direct view of the solar nebula solids that accreted to form Jupiter family comets. The samples collected by the Stardust mission are dominated by high‐temperature materials that are closely analogous to meteoritic components. These materials include chondrule and CAI‐like fragments. Five presolar grains have been discovered, but it is clear that isotopically anomalous presolar grains are only a minor fraction of the comet. Although uncertain, the presolar grain content is perhaps higher than found in chondrites and most interplanetary dust particles. It appears that the majority of the analyzed Wild 2 solids were produced in high‐temperature “rock forming” environments, and they were then transported past the orbit of Neptune, where they accreted along with ice and organic components to form comet Wild 2. We hypothesize that Wild 2 rocky components are a sample of a ubiquitously distributed flow of nebular solids that was accreted by all bodies including planets and meteorite parent bodies. A primary difference between asteroids and the rocky content of comets is that comets are dominated by this widely distributed component. Asteroids contain this component, but are dominated by locally made materials that give chondrite groups their distinctive properties. Because of the large radial mixing in this scenario, it seems likely that most comets contain a similar mix of rocky materials. If this hypothesis is correct, then properties such as oxygen isotopes and minor element abundances in olivine, should have a wider dispersion than in any chondrite group, and this may be a characteristic property of primitive outer solar system bodies made from widely transported components.     

Cover

Cover: Light microscope image of 950 μm‐long Stardust track, C2045,4,178,0,0 (#178), of cometary grains captured in aerogel at 6.1 kms^‐1. Hicks et al. discuss the identifi cation of magnetite in this and other Stardust tracks. The magnetite is interpreted to have formed through water‐rock reaction on the comet Wild 2 parent body. Scale bar = 200 μm. Image courtesy of L. Hicks.     

Sulfur four isotope NanoSIMS analysis of comet‐81P/Wild 2 dust in impact craters on aluminum foil C2037N from NASA’s Stardust mission

Abstract– 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.     

Prediction of the in-situ dust measurements of the stardust mission to comet 81P⧸Wild 2

We predict the amount of cometary, interplanetary, and interstellar cosmic dust that is to be measured by the Cometary and Interstellar Dust Analyzer (CIDA) and the aerogel collector on board the Stardust spacecraft during its fly-by of comet P⧸Wild 2 and during the interplanetary cruise phase. We give the dust flux on the spacecraft during the encounter with the comet using both, a radially symmetric and an axially symmetric coma model. At closest approach, we predict a total dust flux of 10⁶⁰ m⁻² s⁻¹ for the radially symmetric case and 10⁶⁵ m⁻² s⁻¹ for the axially symmetric case. This prediction is based on an observation of the comet at a heliocentric distance of 1.7 AU. We reproduce the measurements of the Giotto and VEGA missions to comet P⧸Halley using the same model as for the Stardust predictions. The planned measurements of interstellar dust by Stardust have been triggered by the discovery of interstellar dust impacts in the data collected by the Ulysses and Galileo dust detector. Using the Ulysses and Galileo measurements we predict that 25 interstellar particles, mainly with masses of about 10⁻¹² g, will hit the target of the CIDA experiment. The interstellar side of the aerogel collector will contain 120 interstellar particles, 40 of which with sizes greater than 1 μm. Furthermore, we investigate the contamination of the CIDA and collector measurements by interplanetary particles during the cruise phase.     

Interpretation of Wild 2 dust fine structure: Comparison of Stardust aluminum foil craters to the three‐dimensional shape of experimental impacts by artificial aggregate particles and meteorite powders

Abstract— New experimental results show that Stardust crater morphology is consistent with interpretation of many larger Wild 2 dust grains being aggregates, albeit most of low porosity and therefore relatively high density. The majority of large Stardust grains (i.e. those carrying most of the cometary dust mass) probably had density of 2.4 g cm^?3 (similar to soda‐lime glass used in earlier calibration experiments) or greater, and porosity of 25% or less, akin to consolidated carbonaceous chondrite meteorites, and much lower than the 80% suggested for fractal dust aggregates. Although better size calibration is required for interpretation of the very smallest impacting grains, we suggest that aggregates could have dense components dominated by μm‐scale and smaller sub‐grains. If porosity of the Wild 2 nucleus is high, with similar bulk density to other comets, much of the pore space may be at a scale of tens of micrometers, between coarser, denser grains. Successful demonstration of aggregate projectile impacts in the laboratory now opens the possibility of experiments to further constrain the conditions for creation of bulbous (Type C) tracks in aerogel, which we have observed in recent shots. We are also using mixed mineral aggregates to document differential survival of pristine composition and crystalline structure in diverse finegrained components of aggregate cometary dust analogues, impacted onto both foil and aerogel under Stardust encounter conditions.