Skip to main content
NCBI home page
NASA Author Manuscripts logoLink to NASA Author Manuscripts
. Author manuscript; available in PMC: 2020 Jan 1.
Published in final edited form as: Meteorit Planet Sci. 2018 Aug 5;54(1):202–219. doi: 10.1111/maps.13182

Comparison of GEMS in Interplanetary Dust Particles and GEMS-like Objects in a Stardust Impact Track in Aerogel

Hope A Ishii 1
PMCID: PMC6350812  NIHMSID: NIHMS984796  PMID: 30713419

Abstract

Comet 81P/Wild 2 dust, the first comet sample of known provenance, was widely expected to resemble anhydrous chondritic porous (CP) interplanetary dust particles (IDPs). GEMS, distinctly characteristic of CP IDPs, have yet to be unambiguously identified in the Stardust mission samples despite claims of likely candidates. One such candidate is Stardust impact track 57 “Febo” in aerogel, which contains fine-grained objects texturally and compositionally similar to GEMS. Their position adjacent the terminal particle suggests that they may be indigenous, fine-grained, cometary material, like that in CP IDPs, shielded by the terminal particle from damage during deceleration from hypervelocity. Darkfield imaging and multi-detector energy-dispersive x-ray mapping were used to compare GEMS-like-objects in the Febo terminal particle with GEMS in an anhydrous, chondritic IDP. GEMS in the IDP are within 3× CI (solar) abundances for major and minor elements. In the Febo GEMS-like objects, Mg and Ca are systematically and strongly depleted relative to CI; S and Fe are somewhat enriched; and Au, a known aerogel contaminant is present, consistent with ablation, melting, abrasion and mixing of the SiOx aerogel with crystalline Fe-sulfide and minor enstatite, high-Ni sulfide and augite identified by elemental mapping in the terminal particle. Thus, GEMS-like objects in “caches” of fine-grained debris abutting terminal particles are most likely deceleration debris packed in place during particle transit through the aerogel.

Introduction

The NASA Discovery-class mission Stardust, launched in 1999 and returned in 2006, was the first sample return mission to a comet (Brownlee et al. Science 2006). The target was comet 81P/Wild 2, a Kuiper Belt object that was diverted into an inner Solar System orbit by an encounter with Jupiter’s gravity. At the time of the Stardust fly-by, Wild 2 had completed only a handful of inner Solar System orbits of the Sun and was releasing gas and dust in jets from localized active regions on the comet surface. Despite the considerable risk of large particle impacts to the spacecraft and sample collector integrity, the spacecraft successfully flew through the comet coma in 2004 at a closest approach of ~150 km. During the fly-by, comet dust particles were captured at a relative speed of 6 km/s in a collector of low-density silica aerogel tiles secured by foil spacers in an aluminum frame. The sample return capsule reentered Earth’s atmosphere in January 2006, and researchers have been studying the samples since, using multiple analytical techniques (e.g. Brownlee et al. 2006, Zolensky et al. 2006, McKeegan et al. 2006, Nakamura et al. 2008, Ishii et al. 2008, Chi et al. 2009, Matzel et al. 2010, Ogliore et al. 2012, Westphal et al. 2014, Frank et al. 2014, De Gregorio et al. 2017, Kearsley 2017, Haas et al. 2017, Wozniakiewicz et al. 2018).

Prior to the return of the Stardust samples from comet 81P/Wild 2, the only comet samples available for study in Earth laboratories were dust particles that, by chance, drifted into Earth-crossing orbits and settled through the atmosphere. These particles, known as interplanetary dust particles (IDPs) due to their long transit between planets before arrival at Earth, are most commonly collected by stratospheric aircraft during long, high altitude flights on collection flags coated with high viscosity silicone oil (Brownlee 1985). Other collection methods have since been explored, including recovery from Antarctic snow, ice, and well water (e.g. Maurette et al. 1991; Dartois et al. 2013; Noguchi et al. 2014; Taylor et al. 1998), high volume air samplers placed in locations with limited terrestrial dust (Wozniakiewicz et al. 2011, 2014; Alesbrook et al. 2017; Ishii et al. 2017; Taylor et al. 2017), and as-yet very limited, silicone oil-free collections by stratospheric flight (Messenger et al. 2015).

Among IDPs, the subset of anhydrous, chondritic (solar composition), porous interplanetary dust particles (CP IDPs) are believed to originate from cometary parent bodies. Many CP IDPs contain crystalline silicate minerals with solar wind-generated amorphized rims and solar flare tracks with densities consistent with ~10,000 year residence times in the inner Solar System (Bradley et al., 1984). These IDPs exhibit anhydrous mineralogy consistent with water present only as ice that has since sublimed away, fine-grained porous aggregate structures consistent with cometary meteor breakup, the highest abundance of presolar grains identifiable by isotope anomalies among all meteoritic materials available for study, high volumes of organics and silicate minerals consistent with cometary infrared spectra, and typically, high average atmospheric entry speed consistent with cometary orbits (Verniani 1969; Sandford and Walker 1985; Flynn 1989; Joswiak et al. 2000; Floss et al., 2006; Bradley 2014; Wooden et al 2017). In addition to Mg-rich olivine, low-Ca pyroxene, disordered refractory carbonaceous material, sulfides and other minor phases, they contain a high volume of amorphous silicates known as GEMS, glass with embedded metals and sulfides.

Based on our knowledge of CP IDPs, the research community’s expectations were that the samples returned from comet 81P/Wild 2 would be similar to CP IDPs. The Stardust mission was the key test of whether CP IDPs originate from Jupiter-family comets. In addition to high abundances of carbon (up to 90% by volume) and isotopically anomalous presolar grains (≥375 ppm for primitive IDPs and as high as 15,000 ppm) (Floss et al., 2006; Busemann et al. 2009), CP IDPs contain two exotic components that distinguish this class of meteoritic materials from others and were therefore primary targets for identification in the Stardust sample. The first are submicron-sized amorphous silicate grains known as GEMS, glass with embedded metal and sulfides (Bradley 1994; Brownlee et al. 2005; Bradley 2014). Oxygen isotopic compositions establish that some are interstellar amorphous silicates which should be preserved in highest abundance in comets (Messenger et al. 2003). At the first meeting of the Stardust Preliminary Examination researchers (Timber Cove Inn, Jenner, CA in July 2006), this author presented transmission electron micrographs and chemical analysis of an object seen in the Stardust sample with the heading “Melted GEMS?” (Fig. 1). Subsequent early publications also referred to possible GEMS (e.g. Brownlee et al. 2006, Keller et al. 2006) as objects that looked tantalizingly GEMS-like continued to be found, while others pointed out differences between the Stardust objects and GEMS in CP IDPs (Zolensky et al. 2006).

Figure 1.

Figure 1.

Open in a new tab

A possible melted GEMS in a NASA Stardust returned sample from comet 81P/Wild 2, NASA designation: FC5-2-5-0-13. (a) Bright field image and (b) high angle annular dark field (HAADF) image show embedded metal and Fe(Ni)-sulfide grains in an amorphous silicate matrix. EDX analysis showed that the inclusions are typically 87 at% Fe with ~6-7 at% Ni and S and the amorphous silicate matrix contains Mg, Al and Ca in 90% SiO2. The measured composition suggests loss of sulfur and inclusion of large amounts of silica aerogel. The rounded textures suggest melting and quenching.

Subsequent laboratory simulations compared with Stardust samples demonstrated, however, that there was an unfortunate consequence of the choice of silica (SiOx) aerogel as the collection medium for the Stardust mission. The silica aerogel combined with the hypervelocity impact capture speed resulted in debris left along impact tracks very similar in morphology and texture to GEMS in CP IDPs (Ishii et al. 2008). Impacts at the Stardust capture speed of 6 km/s of sulfide-bearing grains were shown to result in ablation, melting, abrasion and mixing with melted aerogel (as well as some cometary silicate melt and vapor) that produce glassy objects embedded with sulfide and metal beads. These objects look uncannily like GEMS and are referred to as GEMS-like objects. (Figure 3 in Ishii et al. Science 2008 shows a side-by-side comparison of images of GEMS in an IDP, a Stardust returned sample, and a laboratory hypervelocity impact of pyrrhotite into silica aerogel.) The embedded beads commonly have a rim-core structure with metal (zero to very low S content) on the interior and sulfide on the exterior, sometimes with a gap between them caused by their different coefficients of thermal expansion during cooling (Fig. 2). The rim-core structure likely forms by melting and dispersion of sulfide droplets, accompanied by partial volatilization of sulfur, in friction-melted aerogel and other cometary solids, followed by thermal quenching (Leroux & Jacob, 2013).

Figure 3.

Figure 3.

Open in a new tab

Stardust impact track 57 Febo (C2009,2,57). (a) Transmitted light photomicrograph of the track in aerogel with higher magnification images of the terminal particles inset. High angle annular dark field (HAADF) image of the two Febo TP sections examined in this study, (b) Febo-B-6 (NASA designation C2009,2,57,2,20) and (c) Febo-B-9 (NASA designation C2009,2,57,2,28).

Figure 2.

Figure 2.

Open in a new tab

Bright field TEM image of typical metal core and sulfide rim structure observed on rounded grains embedded in silica glass by light gas gun impacts of pyrrotite (FeS) into silica aerogel at Stardust capture speeds of 6 km/s. The gap between core and rim is due to differential coefficients of thermal expansion on cooling. Analogous core-rim structures are observed in actual Stardust samples and are not observed in GEMS in CP IDPs (Ishii et al. 2008).

The second exotic component of CP IDPs comprises enstatite whiskers and platelets with particular crystallographic habits and axial screw dislocations indicative of vapor-phase growth (Bradley et al., 1983). Several elongated enstatite crystals with whisker-like morphologies have been found in the Wild 2 sample, but their crystal habits and absence of crystallographic evidence of axial screw dislocations are incompatible with vapor phase growth and instead consistent with growth from a melt (Ishii et al. 2008). There has been one report of a CP-IDP-like enstatite whisker with a linear feature suggested to be a screw dislocation (Stodolna et al. 2014). Further electron diffraction is required to establish the nature of a linear feature in a crystal.

As a consequence of the dearth of identifiable GEMS and vapor-phase-grown enstatite whiskers, as well as conspicuously lower carbon content than in CP IDPs, Ishii et al (2008) suggested that comet Wild 2 samples bear more resemblance to meteorites from the asteroid belt than anhydrous CP IDPs also believed to originate from comets, indicating a continuum between asteroids and comets. Subsequent studies have strengthened the links to multiple meteorite classes (e.g. Frank et al. 2014). Ishii et al. (2008) concluded that GEMS may be impossible to identify, even if they were originally present in Wild 2 dust, due to the capture process: “…In Stardust tracks, GEMS-like material was created during capture by melting and intermixing of aerogel with crystalline minerals, including silicates and sulfides.” More recent reviews (Brownlee 2014, Wooden et al. 2017, Westphal et al. 2017) also note the potential significance of, but limitations to, positively identifying bona fide GEMS in Stardust mission samples.

It has been proposed that large terminal particles that were robust enough to remain intact during their deceleration from 6 km/s to standstill in the silica aerogel collection medium, acted to shield fine-grained chondritic material “sheltered in the lee” of the particle during capture (Brownlee et al. 2006), and many researchers hold hopes that a cache of preserved fine-grained CP IDP-like material may yet be located. Some have suggested that fine-grained material is likely preserved (e.g. Gainsforth et al. 2016, Brownlee 2014). In order to assess whether fine-grained material found behind terminal particles in Stardust samples is A) indigenous GEMS-rich material that was shielded from high temperature interaction with aerogel by the terminal particle or B) deceleration debris from the hypervelocity capture process, we report here results of a side-by-side comparison of fine-grained material behind a Stardust terminal particle, “Febo”, and a GEMS-rich, anhydrous, chondritic IDP by high resolution elemental mapping in the (scanning) transmission electron microscope. Portions of the data included here were initially presented at a Meteoritical Society meeting (Ishii and Bradley 2015), and additional data on the GEMS in the IDP discussed here have been reported in a recent journal article (Ishii et al. 2018) and at a Lunar and Planetary Science Conference Meeting (Ishii and Bradley 2018).

Materials and Methods

Stardust track 57 “Febo” (NASA ID: C2009,2,57) in silica aerogel, shown in Figure 3a, is bifurcated and, at the deepest location in the track, contained an ~8 μm long terminal particle. Early analyses found the particle to consist of Fe-sulfide, enstatite and fine-grained material with approximately chondritic composition. It was viewed as a potential cache of CP IDP-like material that was shielded and preserved during capture by the large mineral grain abutting it (Brownlee et al. 2006). Track 57 “Febo” was extracted from its parent silica aerogel tile, flattened and divided in segments at University of Washington (Seattle, WA). The segment containing the terminal particle was embedded in an acrylic resin for ultramicrotomy (Bradley and Brownlee, 1986), and sections were placed on Cu and Au TEM grids with amorphous carbon substrates followed by acrylic removal by chloroform vapor using the procedure developed by Matrajt and Brownlee (2006). Although this procedure removes some indigenous organics along with the acrylic, it leaves all inorganic matter intact. Two non-consecutive, ultramicrotomed sections of the “Febo” terminal particle with adhering fine-grained material and silica aerogel were analyzed by (scanning) transmission electron microscopy. Their official NASA designations are C2009,2,57,2,20 (initially named Febo-B-6) and C2009,2,57,2,28 (initially named Febo-B-9). The samples are referred to by their shorter nicknames, Febo-B-6 and Febo-B-9, in this paper and shown at low magnification in scanning transmission electron microscope (STEM) high angle annular dark field (HAADF) images in Figures 3b and 3c.

For comparison, an ultramicrotomed thin section of a GEMS-rich, anhydrous, chondritic IDP, U2-17B19, was analyzed in tandem with the ultramicrotomed sections of the Febo terminal particle (Fig. 4). U2-17B19 shows only minor alteration on its exterior by atmospheric entry and limited silicone oil contamination (also concentrated near the exterior), presumably because it is an unusual low porosity IDP with organic carbon matrix preventing the pervasive silicone oil penetration into the interior sometimes observed in other CP IDPs. Such IDPs are called chondritic filled (CF) IDPs and are otherwise identical to CP IDPs ((Brownlee et al. 1982). In the ultramicrotomed section analyzed, IDP U2-17B19 is chondritic, carbon- and GEMS-rich and ~10 μm in length. The IDP was Pd-coated for initial imaging by SEM, and it was subsequently embedded in elemental sulfur and the sulfur bead enclosed in epoxy for ultramicrotomy using the method of Bradley (Bradley et al 1993). The elemental sulfur subsequently sublimes to leave a “naked” section of the IDP, physically separated from the epoxy embedding medium.

Figure 4.

Figure 4.

Open in a new tab

Anhydrous and chondritic IDP U2-17B19. (a) Secondary electron image. (b) HAADF image of a 70-80 nm thick, ultramictromed section. The bright rim on the lower left edge is a Pd coating that was applied prior to particle embedding and ultramicrotomy to enable secondary electron imaging. Dark gray areas correspond to organic carbon matrix. Brighter regions correspond to GEMS and minor silicate and sulfide mineral grains.

Two FEI Titan (scanning) transmission electron microscopes ((S)TEMs) with ChemiSTEM technology were used to analyze the samples. One is an 80-200 kV FEI Titan ChemiSTEM at Oregon State University (Corvallis, OR). The other is a 60-300 kV FEI Titan ChemiSTEM at the Molecular Foundry’s National Center for Electron Microscopy (NCEM) at Lawrence Berkeley National Lab (Berkeley, CA). The ChemiSTEM technology is a multi-Si-drift-detector energy dispersive x-ray (multi-SDD EDX) detector array that provides high solid angle acceptance for rapid, high signal-to-noise elemental mapping. The OSU Titan ChemiSTEM has a high brightness FEG, and both Titan ChemiSTEMs have 4 Bruker SDDs providing approximately 0.7 steradians solid angle and 140 eV energy resolution (at Mn Kα). Sample mapping was performed at 200 kV at beam currents of ≤ 750 pA and usually ~400 pA. These beam currents are an order of magnitude lower than those commonly used to perform EDX mapping in a TEM with a single EDX detector, a key benefit of the multi-SDD EDX approach. Acceptable working beam current was established by an assessment on another anhydrous CP IDP of beam damage effects as assessed by changes to shape or to elemental abundances as a function of electron probe current. The ChemiSTEM electron probe can be focused to less than a nanometer in diameter but was defocused slightly to reduce electron current density at each pixel location without degrading image resolution for a given pixel spacing. Figure 5 shows a (S)TEM HAADF image collected prior to and after 2.5 hours of mapping over most of the region shown. No shape changes, distortions of relative grain positions or element abundances were detected in the GEMS or silicate regions.

Figure 5.

Figure 5.

Open in a new tab

HAADF images (a) before and (b) following completion of a 2.5 hour multi-detector EDX compositional map showing no visible changes to the sample. No loss of moderately volatile elements was detected.

Multi-spectral images were collected using Esprit 1.9 software (Bruker Corporation) with a full x-ray fluorescence spectrum at each pixel. A high angle annular dark field (HAADF) image was collected simultaneously. Mapping of fine-grained material in U2-17B19 and in Febo-B-6 and Febo-B-9 was performed with pixel spacing of 3.3 nm over fields of view typically ~2.5 μm on a side. Larger area maps were collected with 12 nm pixel spacing and, for Febo-B-6, over a 9.6 μm field of view. Data were collected in iterative map frames with EDX spectra accumulated in each pixel to create a datacube where the 1st and 2nd dimensions correspond to the position within the map area, and the 3rd dimension is the x-ray intensity as a function of energy. The Esprit software allows presentation of element distributions in color-coded maps generated by assigning an intensity level at each pixel based on the integrated counts in an energy window set around characteristic K-edge x-ray energies for each element. The intensity maps are plotted in user-specified colors to indicate the spatial distribution of each element. Element maps presented here include 3-pixel averaging, unless otherwise noted, to improve visualization. Element maps are for visualization only and are not accurate representations of quantitative composition. Due to limited signal-to-noise in an individual pixel, background fitting and removal is generally not possible, so individual element maps can be occasionally misleading: In regions where high Z elements generate higher Brehmsstrahlung background, or when nearby higher abundance elements create an overlap in peak tails, there may be extra counts in the energy window corresponding to a given element.

To circumvent this limitation of mapping counts in energy windows, regions of interest that contain many pixels are defined within a map, and the spectra in those regions are summed to generate sum spectra with sufficient signal-to-background, fit, and quantified to produce elemental analyses. Regions were defined that contained GEMS in IDP U2-17B19 and GEMS-like-objects in the Febo terminal particle sections, and compositions were quantified using the Cliff-Lorimer approximation (Cliff and Lorimer, 1975) with major element k-factors confirmed by analyses of silicate and sulfide minerals. Compositions for major and minor rock-forming elements, excluding O, are reported in element atomic percent normalized to 100 at.% in Table 1. Measured O atomic percent values for a pyroxene crystal in the IDP were within 5% of the stoichiometric value indicating low levels of x-ray self-absorption. Uncertainties in quantified element abundances depend on abundance as well as the number of pixels summed. Relative uncertainties (1-σ) for GEMS and non-GEMS aggregate regions in the IDP are typically ~0.5% or Si, 3% for Mg and Fe, 4% for S and 5% for Al, Ca, Ni. Relative uncertainties for GEMS-like object regions in Febo-B-6 and Febo-B-9 are typically ~1% for Si, 3% for Fe and S, 6% for Al, and 7% for Ni. They range from 3% to 50% for Mg and from 9% to 152% for Ca, depending on abundance, in the GEMS-like object regions in Febo. In mineral aggregate regions in Febo, relative uncertainties are ~4% for Mg and 9% for Ca. Trace levels of Na, P, Cl, K, Ti, Cr and Mn were detected in IDP U2-17B19 GEMS, and occasional traces of P, Cr and Mn were detected in the Febo sections, but they are not reported here due to high levels of uncertainty and some element peak overlaps with system peaks.

Table 1.

Normalized elemental compositions and uncertainties (1σ) in atomic percent of regions of GEMS, GEMS-like object and coarse-grained aggregates in chondritic IDP U2-17B19, Febo-B-6 and Febo-B-9.

Region Mg 1σ Al 1σ Si 1σ S 1σ Ca* 1σ Fe 1σ Ni 1σ
GEMS regions in CP IDP U2-17B19
1 22.7 0.7 4.27 0.18 40.4 0.1 12.2 0.4 1.53 0.08 17.4 0.5 1.65 0.07
2 19.6 0.6 2.75 0.14 30.7 0.1 10.8 0.4 1.62 0.09 33.0 1.0 1.60 0.07
3 21.8 0.7 3.18 0.15 37.2 0.1 2.82 0.13 2.04 0.10 31.4 1.0 1.49 0.07
4 31.9 1.0 3.43 0.15 43.0 0.1 7.84 0.28 1.73 0.09 11.3 0.4 0.83 0.05
5 23.4 0.8 5.36 0.22 43.8 0.2 6.93 0.26 2.20 0.11 16.3 0.5 2.00 0.09
6 30.4 1.0 3.79 0.17 43.5 0.2 6.69 0.25 2.14 0.11 12.6 0.4 0.90 0.06
7 24.4 0.8 4.70 0.19 37.1 0.1 11.0 0.4 1.87 0.09 19.3 0.6 1.58 0.07
8 26.7 0.9 4.28 0.18 41.3 0.2 10.0 0.3 1.38 0.08 15.1 0.5 1.27 0.06
9 23.9 0.8 4.47 0.20 41.1 0.2 6.65 0.25 1.69 0.09 20.5 0.6 1.67 0.08
10 25.1 0.8 2.96 0.14 33.5 0.1 17.6 0.6 1.17 0.07 18.3 0.6 1.39 0.06
11 29.1 0.9 2.56 0.10 35.1 0.2 3.65 0.16 1.56 0.09 26.9 0.8 1.11 0.06
12 26.1 0.8 4.82 0.20 39.1 0.1 11.0 0.4 2.35 0.11 15.5 0.5 1.06 0.06
13 32.9 1.1 3.86 0.17 42.2 0.2 3.58 0.16 1.04 0.07 15.1 0.5 1.28 0.07
14 22.5 0.7 3.84 0.17 38.1 0.2 13.6 0.5 1.67 0.09 18.8 0.6 1.47 0.07
15 25.8 0.8 5.44 0.23 46.9 0.2 7.54 0.28 1.61 0.09 12.0 0.4 0.76 0.05
16 32.5 1.0 3.26 0.15 40.2 0.2 7.82 0.28 1.75 0.09 13.2 0.4 1.26 0.06
GEMS-like object regions in Stardust Febo-B-6
1 0.48 0.08 2.66 0.14 37.8 0.2 27.9 0.9 nd nd 30.4 0.9 0.74 0.05
2 0.51 0.06 1.68 0.10 24.5 0.2 24.8 0.8 0.06 0.04 47.7 1.5 0.78 0.07
3 2.33 0.12 2.88 0.15 37.5 0.2 26.7 0.9 0.14 0.01 29.4 0.9 1.10 0.10
4 7.10 0.33 3.04 0.19 59.9 0.5 12.9 0.5 0.20 0.06 14.7 0.5 2.13 0.13
5 2.53 0.16 3.47 0.18 44.1 0.3 23.4 0.8 0.10 0.05 24.7 0.8 1.61 0.09
6 4.37 0.20 1.85 0.12 31.1 0.2 30.5 1.0 0.40 0.05 30.1 0.9 1.71 0.09
7 1.06 0.10 1.49 0.10 16.3 0.1 38.8 1.2 nd nd 41.5 1.3 0.91 0.06
8 0.54 0.10 2.40 0.15 15.0 0.2 39.1 1.2 nd nd 37.9 1.2 5.03 0.20
9 0.30 0.07 1.56 0.11 17.6 0.2 37.8 1.2 nd nd 41.1 1.3 1.55 0.08
10 1.73 0.13 3.49 0.18 44.3 0.3 23.6 0.8 nd nd 26.0 0.8 0.78 0.06
12 3.44 0.28 3.64 0.28 62.9 0.9 13.4 0.6 0.34 0.10 15.7 0.6 0.64 0.10
13 0.14 0.07 2.09 0.14 27.6 0.3 33.6 1.1 nd nd 36.0 1.1 0.59 0.06
14 1.99 0.14 2.35 0.14 26.0 0.2 33.3 1.1 nd nd 35.6 1.1 0.80 0.06
15 1.24 0.12 1.73 0.13 19.4 0.3 38.4 1.2 nd nd 35.0 1.1 4.14 0.17
GEMS-like object regions in Stardust Febo-B-9
1 0.34 0.07 2.12 0.12 19.0 0.2 38.9 1.2 nd nd 38.9 1.2 0.82 0.05
2 8.79 1.43 3.23 0.23 37.8 0.5 23.7 0.5 nd nd 25.7 0.6 0.83 0.13
3 0.26 0.07 1.24 0.10 8.1 0.1 46.2 1.4 nd nd 43.8 1.4 0.46 0.05
4 1.89 0.12 2.70 0.14 22.7 0.2 36.2 1.1 nd nd 35.8 1.1 0.74 0.05
5 0.34 0.08 2.02 0.13 22.8 0.2 36.7 1.2 nd nd 37.1 1.2 1.01 0.07
6 2.94 0.16 2.74 0.15 21.0 0.2 36.0 1.1 nd nd 36.1 1.1 1.18 0.07
7 1.07 0.12 2.99 0.18 22.1 0.3 36.8 1.2 nd nd 36.1 1.1 0.92 0.08
8 7.42 0.30 4.53 0.20 29.8 0.2 28.9 0.9 0.23 0.05 28.7 0.9 0.54 0.05
10 0.46 0.08 3.08 0.17 19.8 0.2 38.3 1.2 nd nd 37.6 1.2 0.69 0.06
11 0.21 0.08 1.79 0.13 19.3 0.3 38.2 1.2 nd nd 39.3 1.2 1.15 0.08
Coarse-grained aggregates in CP IDP U2-17B19
A1 23.4 0.8 3.25 0.15 33.5 0.1 17.8 0.6 2.31 0.10 18.0 0.6 1.83 0.08
A2 29.9 0.9 2.69 0.13 29.0 0.1 17.6 0.6 0.69 0.05 19.0 0.6 1.15 0.06
A3 18.6 0.6 2.57 0.14 16.7 0.1 7.97 0.29 1.10 0.07 51.2 1.6 1.81 0.08
Coarse-grained aggregates in Febo-B-6
11 13.3 0.5 3.93 0.18 42.5 0.2 17.8 0.6 0.51 0.06 21.0 0.7 0.93 0.06
16 21.8 0.7 4.56 0.19 35.6 0.2 15.5 0.5 1.34 0.08 20.4 0.6 0.76 0.05
Coarse-grained aggregates in Febo-B-9
9 30.4 1.0 3.29 0.15 33.3 0.1 13.4 0.4 0.3 0.0 18.2 0.6 1.15 0.06
Open in a new tab
*

nd = non-detect, below detection limits

Measured element abundances of Mg, Al, Si, S, Ca, Fe and Ni, normalized to 100%, and ratioed to chondritic abundances (CI) of the same elements, also normalized to 100% are also presented here in graphical form. For example, for magnesium, the ratio reported is Mgsample/MgCI. Element abundances are also reported ratioed to both Si abundance and CI abundance in Appendix A. For high abundance elements, uncertainties in the reported ratios are typically dominated by the uncertainty in the CI chondrite abundances (Palme et al. 2014). These are 3% for Si, 4% for Mg and Fe, 5% for S, 6% for Al and Ca, and 7% for Ni.

New Details Revealed by EDX Spectral Mapping

Beyond the immediate goal of comparing GEMS-like material behind a Stardust terminal particle with GEMS in an anhydrous, chondritic IDP, high signal-to-noise EDX spectral mapping with multi-SDD detectors has revealed new details about the Febo terminal particle. The initial analyses by conventional analytical TEM had identified three major components: pyrrhotite (FeS), enstatite (MgSiO3) and ~chondritic-composition fine-grained material (Brownlee et al. 2006). An EDX spectral map collected over the entire Febo-B-6 section of the particle rapidly shows additional details not readily observed using standard TEM methods. Figure 6 shows a multi-element map over the same area shown in the HAADF STEM image in Figure 3b. Several different mineral regions are visible. By summing pixels in these regions, we found that, in addition to pyrrhotite (Fe:Ni=98:2), regions with higher Ni content (Fe:Ni=76:24) are also present. Less volume of enstatite is present is this section of the Febo terminal particle than in the section of the same particle described in Brownlee et al. (2006), and its composition is (Mg0.89Ca0.03Cr0.01Mn0.02Fe0.05)SiO3 or Fo94.7. Micron-sized enstatite grains show a shatter pattern typical of microtomed sections (as does the large sulfide grain), but small rounded ~100-200 nm enstatite grains, or “beads”, are also present. In addition to enstatite, small rounded beads of a Ca-bearing pyroxene, augite, ~50-150 nm in size, are also present in the fine-grained material with a composition of (Ca0.34Mg0.48Fe0.10Al0.07)SiO3. An unexpected high-Al phase is also observed, likely aluminum oxide from the collector but possibly also a silicate, that was heavily damaged during deceleration in the aerogel; electron diffraction was indeterminate. Its composition is consistent with Al2O3 or with an aluminosilicate (Al2SiO5) if additional SiO2 was incorporated from aerogel. It is present as a ~0.5 micron-sized irregularly-shaped mass and also as ~50-200 nm rounded beads. We note that Al content in the Stardust aerogel is <2.5 ppm (Tsou et al. 2003). The rounded shapes of several components in the terminal particle are suggestive of either melt-quench processing caused by hypervelocity capture or of abrasive rounding by interaction with the aerogel – or both. Melt-quench processing would further imply incorporation of other glass-forming elements in addition to Si into deceleration debris.

Figure 6.

Figure 6.

Open in a new tab

Overlaid element maps of Mg, Al, Ca, Fe and Ni of the area corresponding to the HAADF STEM image in Figure 3a. To better show phases present, 5-point pixel averaging was applied.

Comparison of Fine-Grained Material in Stardust Particle and GEMS in an IDP

Visual comparisons of the HAADF STEM images from the two sections of the Febo terminal particle and from the section of the anhydrous, chondritic IDP U2-17B19, shown in Figures 3 and 4, respectively, show gross similarities in the textures of the fine-grained material. However, the multi-SDD EDX mapping of similar-sized regions and at the same spatial resolution reveal key differences. Figure 7 shows HAADF STEM images of regions of fine-grained material and corresponding color-coded element maps in Febo-B-6 and CP IDP U2-17B19. Differences between the IDP fine-grained material and the Febo fine-grained material are readily observable: The IDP fine-grained material is mostly blue-toned Mg-rich silicate with some green-toned Fe-rich sulfides in contrast to Track 57 fine-grained material, which is mostly green-toned Fe-rich sulfides. In addition, the IDP has a much finer structure in element distributions than the Febo fine-grained material. GEMS in the IDP are generally rounded with distinct edges, whereas the GEMS-like objects in the Febo samples are not rounded, show some preferential texture in their long axis, and have less well-defined edges. Several coarser-grained equilibrated aggregates with subhedral grain shapes (in the upper right corner of Fig. 7d, for example) are present in the IDP. Coarser-grained aggregates are also present in the Febo samples, but only rounded grain shapes were observed and, instead of being distributed throughout the fine-grained material as in the IDP, they tend to be concentrated near the large mineral grains of the terminal particle. Major element Si (not shown in Figure 7) is coincident with Mg in the IDP; in the Febo samples, Si is also coincident with Mg in silicates, but it is also present in and around Fe-S-O-containing regions. The predominance of Fe-S-rich material and low abundance of Mg-silicate are consistent with the fine-grained Febo material having formed by ablation of the adjacent Fe-sulfide terminal particle and mixing with aerogel. Like snow packing around a rock rolling through snow, this debris may have adhered to the terminal particle and been carried down the track with it.

Figure 7.

Figure 7.

Open in a new tab

Side-by-side comparisons of fine-grained material in Febo-B-6 (top) and chondritic IDP U217B19 (bottom) at the same scales. (a) HAADF STEM image of Febo-B-6 and (b) corresponding overlaid element maps for Mg, S, Ca, Fe and Ni. (c) HAADF STEM image of U2-17B19 and (d) corresponding overlaid element maps. Color code for elements is given on right. Major element Si is coincident with Mg in U2-17B19.

To quantify composition differences between GEMS in the anhydrous, chondritic IDP and GEMS-like-objects in the Febo fine-grained material, 16 regions containing GEMS and 3 regions containing equilibrated aggregates and other non-GEMS assemblages were defined in IDP U2-17B19, as shown in Figure 8; 14 regions containing GEMS-like-objects and 2 regions (#11 and #16) containing aggregates of ~100-150 nm mineral grains were defined in Febo-B-6, as shown in Figure 9; and an additional 10 GEMS-like-objects and 1 aggregate of mineral grains (#9) were defined in Febo-B-9, as shown in Figure 10. GEMS in the IDP are generally rounded in shape and well-defined and, thus, readily outlined. Some regions may include multiple GEMS, but this is not expected to impact composition comparisons between GEMS in the IDP and GEMS-like-objects in the Febo terminal particle sections. For this work, no effort was made to exclude GEMS that were altered by atmospheric entry. Boundaries for GEMS-like-objects in the Febo fine-grained material were selected to be of similar sizes since they were not rounded objects and were less well-defined. Elemental compositions were determined for each region and are given in Table 1.

Figure 8.

Figure 8.

Open in a new tab

Regions defined in chondritic IDP U2-20B19 for compositional analysis are a) 16 regions containing GEMS and b) an additional 3 regions containing equilibrated aggregates and other non-GEMS assemblages. The multi-element map is from Fig. 7d.

Figure 9.

Figure 9.

Open in a new tab

Regions defined in the Febo-B-6 thin section of the track 57 Febo terminal particle for compositional analysis are a) 14 regions of GEMS-like-objects and b) 2 regions containing aggregates of mineral grains (#11, #16). Color scheme is same as Figure 7.

Figure 10.

Figure 10.

Open in a new tab

Regions defined in the Febo-B-9 thin section of the track 57 Febo terminal particle for compositional analysis are a) ten regions of GEMS-like-objects and b) one region (#9) containing aggregates of mineral grains. Color scheme is same as Figure 7.

To compare the GEMS regions in the IDP with the GEMS-like object regions in the fine-grained material associated with the Stardust Febo terminal particle, the compositions in Table 1 were plotted relative to their CI (chondritic) abundances (Palme et al. 2014). (Plots of element abundances relative to both Si and CI and a brief discussion of issues with Si as a ratio element are included in Appendix A.) Figure 11a shows the element/CI ratios for Mg, Al, Si, S, Ca, Fe and Ni for the 16 regions containing GEMS in IDP U2-17B19. The GEMS all fall within 3× CI with two exceptions which show more depletion in S, the most volatile element and most likely to be lost during atmospheric entry. The Si ratios also cluster fairly closely about 1. Figures 11b and 11c show the same element/CI ratios for 14 GEMS-like objects in Febo-B-6 and 10 GEMS-like objects in Febo-B-9. In all of the GEMS-like objects, elements fall outside of the 3× CI boundaries. Ca is below the detection limit in 8 of the 14 GEMS-like objects in the Febo-B-6 thin section and in 9 out of the 10 GEMS-like objects in Febo-B-9 thin section despite the presence of some Ca-bearing augite in the thin sections (e.g. Fig. 10). In the Febo sections, in general, Mg and Ca are strongly and variably depleted, and Fe and S tend to be elevated. Al and Ni show some variability, but Al tends to track Si and Ni tends to track Fe. (The exceptions in Febo-B-6 where Ni ratios are higher than Fe ratios appear to include some of the higher-Ni iron sulfide phase observed (Fig. 6). GEMS-like objects with higher Si ratios tend to have lower Fe and S ratios, and those with lower Si ratios tend to have higher Fe and S ratios. Thus, compositions of GEMS-like-objects in the fine-grained material associated with the Stardust Febo terminal particle are not consistent with chondritic material. Instead, they are consistent with a mixture of silica aerogel, Fe(Ni) sulfide and variable (minor) amounts of Mg- and Ca-bearing silicates.

Figure 11.

Figure 11.

Open in a new tab

Element-to-CI ratios for Mg, Al, Si, Ca, Fe and Ni for a) 16 regions containing GEMS in anhydrous, chondritic IDP U2-17B19, b) 14 regions containing GEMS-like-objects in Febo-B-6, c) 10 regions containing GEMS-like-objects in Febo-B-9, d) 3 regions containing equilibrated aggregates and other non-GEMS aggregates in U2-17B19, e) 2 regions containing aggregates of minerals in Febo-B-6, and f) 1 region containing an aggregate of minerals in Febo-B-9. Solid black lines indicate the upper and lower boundaries for 3× CI. Colored lines that connect data points for each sample are to guide the eye.

Figure 11d shows the same element ratios for three non-GEMS aggregate regions in the IDP. It is interesting to note that these also fall within 3× CI. While some or all may possibly have formed from GEMS precursors, it demonstrates that a mixture of mineral grains can produce ~chondritic compositions of major and minor elements. Figures 11e and 11f show the same element ratios for the regions containing aggregates of ~50-200 nm sized mineral grains in the Febo terminal particle fine-grained material. Several of these also fall within ~3 × CI.

Examination of the non-GEMS-like regions 11 and 16 in Febo-B-6 shows that they are aggregates of 150-200 nm bits of partially-oxidized sulfide and enstatite, the same minerals that comprise the larger crystals in the adjacent terminal particle, as well as some Ca-bearing pyroxene (augite). These 50-200 nm components are orders of magnitude larger in size than the components of GEMS and closer to the sizes of components of equilibrated aggregates; however, their chemical match with larger mineral grains strongly suggests they originate as fragments from those larger grains. The augite has the same composition throughout the Febo sections. Although large mineral grains of augite are not seen in Febo-B-6 or Febo-B-9, it is likely that larger fragments of augite are present elsewhere in Track 57. These data demonstrate that near-chondritic compositions, like these observed in the Febo terminal particle fine-grained material, can result from averaging of compositions of multiple mineral fragments.

Zolensky et al. (2008) pointed out that the high silica content of the “glass” in many of the GEMS-like objects analyzed suggest a mixture of aerogel and molten comet particle; however, the degree of aerogel incorporation varies greatly from object to object. In Ishii et al. 2008, we considered whether the GEMS-like material in Stardust tracks might be cometary GEMS intermixed with aerogel. For the GEMS-like objects analyzed in that study, we found that nonvolatile-element atomic ratios (excluding Si and O with which cometary material may have been diluted), were inconsistent with chondritic compositions. The Ishii et al. (2008) paper also showed that characteristic metal-sulfide inclusions are formed when sulfide minerals are involved in hypervelocity impact capture in silica aerogel. Despite the statement that “GEMS-like material was created during capture by melting and intermixing of aerogel with crystalline minerals, including silicates and sulfides”, some authors have since interpreted Ishii et al. (2008) too narrowly as showing that impact-generated GEMS-like objects are formed from the interaction of silica aerogel and cometary sulfides, alone, excluding silicates (e.g. Rietmeijer 2009; Gainsforth et al. 2016). However, it is important to consider that the exteriors of the Wild 2 cometary dust grains, including exposed silicate minerals, are estimated to have experienced temperatures exceeding 2000K during their capture at ~6 km/s into the aerogel (Brownlee et al. 2006). Most cometary silicate minerals have melting point temperatures in that range and, thus, likely also experienced capture abrasion, ablation, melting and mixing, which has been recognized by many researchers. Rietmeijer (2009), for example, notes that “some fraction of the indigenous comet silicates were melted, were perhaps even vaporized.” In this study, we show that approximately chondritic compositions can be obtained from aggregates of 50-200 nm sized mineral fragments of minerals present in larger “grains” in the impacting particle. Thus, it follows that near-chondritic compositions (excluding Si and O) in glassy GEMS-like objects may also result from aggregated mixtures cometary minerals that were ablated, melted and abraded during hypervelocity capture in silica aerogel and are not necessarily diagnostic of bona fide, indigneous GEMS or a GEMS precursor. It is not particularly surprising, since cometary dust may be expected to be chondritic on average, that its deceleration debris might also be chondritic (excepting Si and O).

It is especially noteworthy that fine-grained glassy debris containing metal and sulfide beads that mimic GEMS are found almost exclusively in tracks that are optically dark and that their terminal particle(s) generally contain sulfide. Tracks that are optically clear are most commonly associated with silicate mineral terminal particles and no sulfide or metal. This author knows, anecdotally, of only one exception for which the impact track was optically clear but the terminal particle contained significant sulfide. That sulfide may have been protected from ablation and abrasion, if it was initially surrounded by other more robust minerals until it slowed sufficiently in the aerogel to leave minimal debris. This observation supports the assertion that the characteristic nanoscale core-mantle beads of Fe metal and sulfides in GEMS-like objects are formed during deceleration of larger iron sulfide grains in the aerogel.

In addition to the GEMS-like object found in a Stardust track in Figure 1, Gainsforth et al. (2016) reported on a rounded, glassy object that they nicknamed Daisy that contains Fe(Ni) metal and sulfide and having a GEMS-like composition. The authors concluded that, since Daisy contains chondritic Mg/Si and Fe/Si, no excess Si, and volatile Na and K, it is unlikely to have formed by capture heating. They noted that Daisy is 3× enriched in Na/Si and slightly depleted in K/Si relative to the surrounding material, although the K/Si level is 1.5× chondritic. They conclude that Daisy is a bona fide GEMS grain. While the GEMS-like composition is certainly compelling, we find that the morphology of Daisy indicates that it has, at a minimum, experienced sufficient heating to round it and mobilize sulfides. Since the host track has as its terminal particle a 15 μm diameter pyrrhotite that is next to the fine-grained material containing the GEMS-like object, Daisy, it is exceedingly difficult to unambiguously identify Daisy as a bona fide GEMS grain. Indeed, its position, lodged in a field of deceleration debris, makes it somewhat suspect.

Presence of Gold in the Stardust Fine-Grained Material

Another piece of evidence supports formation of the fine-grained material adjacent to the Stardust track 57 Febo terminal particle by hypervelocity capture: Gold! EDX summed spectra from regions in Febo-B-9 show high abundances of Au, ~40,000-50,000× CI abundances. See Figure 12. Gold has been previously observed in Stardust samples in aerogel and is attributable to contamination from an autoclave used in aerogel processing that left Au near the aerogel surface that was entrained by the cometary particle and carried down the deceleration track (Brennan et al. 2007, Rietmeijer 2016). The presence of Au in the GEMS-like objects implicates aerogel tile incorporation and supports their identification as deceleration debris.

Figure 12.

Figure 12.

Open in a new tab

Energy dispersive x-ray spectroscopy of a region containing GEMS-like objects in Febo-B-9 (C2009,2,57,2,28) shows the presence of gold (Au).

Summary and Conclusions

High resolution, multi-SDD EDX mapping in scanning transmission electron microscopes was applied to two ultramicrotomed sections of a terminal particle from an impact track of comet dust in silica aerogel returned by the Stardust mission and to one section of a chondritic interplanetary dust particle. This mapping technique provides elemental distributions with resolutions down to a few nanometers, rapidly and in a non-destructive manner, for holistic imaging of large areas of TEM samples. A key advantage to this approach is the elimination the inherent user-bias in selecting an area of interest for chemical analysis. As a result, multiple mineral phases present in the Febo terminal particle are apparent as chemically distinct phases, several of which had not been noted in the initial analyses by conventional methods. In addition to enstatite and Fe(Ni) sulfide, minor augite, Ni-rich Fe-sulfide and an Al-rich phase, likely heavily damaged oxide from the collector, were identified.

Through the side-by-side comparison of the GEMS-rich, anhydrous, chondritic IDP, U2-17B19, with the fine-grained GEMS-like material behind the iron-sulfide-and-enstatite terminal particle in the Stardust Track 57, Febo, a number of insights are gained. Although the fine-grained GEMS-like objects behind Febo terminal particle is superficially somewhat similar to GEMS, there are critical differences: GEMS have distinct, rounded shapes, whereas the GEMS-like objects in the Febo samples are not rounded, have less well-defined boundaries and show preferred texture. The GEMS-like objects also do not have as fine a structure in element distributions as GEMS. The compositions of the GEMS-like objects are not chondritic and show large depletions in Mg and Ca and small enrichments in S and Fe. It is possible, though highly unlikely given our current understanding of impact capture in silica aerogel, that the GEMS-like objects observed near the Febo terminal particle are fines, captured intact, with an amorphous silicate GEMS-like component that is compositionally distinct from GEMS in anhydrous, chondritic IDPs. The observed element depletion/enrichment profiles are consistent, however, with the GEMS-like objects having formed in situ as deceleration debris generated by ablation, melting and mixing of SiOx aerogel and the crystalline minerals (Fe-sulfide, enstatite and, likely, augite) that constitute the bulk of the mass of the Wild-2 material at the terminus of Track 57. This is the more likely explanation of their origin. Finally, the presence of Au in the GEMS-like objects points to incorporation of contamination from the silica aerogel tile into the GEMS-like objects, consistent with their identification as deceleration debris created by the hypervelocity capture of comet Wild 2 dust into the aerogel.

Also observed among the GEMS in IDP U2-17B19 were equilibrated aggregates with subhedral grain boundaries on mineral components. Within the Febo fine-grained material are also aggregates of coarse-grained minerals. However, the enstatite and sulfide within them are compositionally identical to the large mineral grains comprising the Febo terminal particle, and they reside in close proximity to those mineral grains. The augite within them has the same composition throughout the Febo sections analyzed. We infer that they are coarse-grained aggregates in the Febo samples of fragments from larger mineral grains. The compositions of those aggregates demonstrate that approximately-chondritic compositions in major and minor elements can be obtained by averaging composition of multiple mineral fragments and, therefore, approximately-chondritic compositions in GEMS-like objects are not uniquely diagnostic of bona fide, indigenous GEMS or GEMS precursors.

Finally, the correlation between iron sulfide in terminal particles and the presence of nanoscale beads of Fe metal and sulfides with core-mantle structure in GEMS-like objects points to their formation during deceleration of iron sulfide grains in the aerogel. These beads may be incorporated into GEMS-like objects formed with silica aerogel alone or may also incorporate ablated, melted and/or abraded fragments of cometary silicates.

In conclusion, the vast majority of GEMS-like-objects are most likely to be aerogel-rich deceleration debris, and caches of fine-grained material thought to have been “entrained” behind robust terminal particles are generally caused by packing of deceleration debris and aerogel next to an (initially irregularly-shaped) comet dust particle as it travels through the aerogel, much as snow packs tightly against a rolling object. As suggested by Ishii et al. (2008), if they were present, any GEMS that were indigenous to comet Wild 2 would, in all likelihood, have been destroyed by the capture process or diluted and comminuted with other components beyond our ability to recognize them. Nonetheless, fine-grained debris captured in track walls and microcracks emanating from them should be investigated by EDX mapping and other methods to explore potential survival of bona fide GEMS there. As reported by Velbel and Harvey (2009), indigenous compositional attributes may be best preserved in the uppermost regions of Stardust impact tracks.

Many researchers have sought evidence for GEMS in the comet 81P/Wild 2 returned sample, and several have claimed to have found likely GEMS, GEMS precursors or capture-modified GEMS (e.g. Brownlee et al. 2006, Stodolna et al. 2013 & 2014, Gainsforth et al. 2016). Since the Stardust mission’s method of capture of comet dust into silica aerogel at hypervelocity is highly problematic for detection of GEMS, such claims must be held to an exceptional standard of proof. Knowledge of the other minerals present in the initial impact is shown here to be especially critical to deconvolving the effects of hypervelocity capture on the composition and make up of the impact debris. Curatorial record-keeping plays a key role in ensuring that this knowledge is available to researchers who are frequently studying a small portion of a much larger impact track.

Acknowledgements

We thank D. Brownlee and G. Matrajt for preparing the Stardust samples, D. Joswiak for discussions, P. Eschbach (Oregon State University) and K. Bustillo (National Center for Electron Microscopy, Molecular Foundry at Lawrence Berkeley National Laboratory) for assistance with Titan ChemiSTEM mapping. J.P. Bradley provided valuable expert input to the manuscript. Thanks also go to G. Flynn for a most helpful review and to Associate Editor, D. Brownlee, for his suggestions, all of which served to improve the manuscript. This research was funded by the NASA Laboratory Analysis of Returned Samples (LARS) Program grant NNH11AQ79I and the NASA Emerging Worlds Program grant NNX16AK41G, both to PI Hope Ishii. Work at the Molecular Foundry was supported by the Office of Science, Basic Energy Sciences, U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Appendix A

Historically, element abundances in IDPs measured by TEM have typically been reported relative to both Si and to CI abundances (e.g. Rietmeijer 1998) among many, many others). Comparing element-to-Si ratios between a measured sample and CI is logical when Si can be assumed to be present as the major rock-forming element and not as a significant contaminant. However, there is a ubiquitous and large excess of Si in the Stardust returned samples captured in aerogel, and silicone oil contributes some excess Si in stratospheric IDPs like U2-17B19. These extra sources of Si serve to artificially depress element-to-Si ratios (Bradley et al. 2011). As a result, element abundance ratios to CI are calculated in the main paper without including Si as a ratio element. For these reasons, some other authors have chosen to use Mg as a ratio element (Schramm et al. 1989), and those using using synchrotron sources not sensitive to lower Z elements like Si have chosen to use Fe as the ratio element (e.g. Kehm et al. 2002).

In Figure A1, the same data are presented with Si as a ratio element to allow direct comparisons with historical data. For example, the Mg ratio is given by (Mgsample/Sisample)/(MgCI/SiCI). This forces the Si ratio to always be equal to 1. The major conclusions are not impacted; Mg and Ca are strongly and variably depleted and S and Fe tend to be enriched in the GEMS-like-objects in Febo-B-6 and Febo-B-9, consistent with a mixture of silica aerogel, Fe(Ni) sulfide and minor amounts of silicates. Near chondritic compositions can be formed by aggregates of mineral grains. When Si is forced to sit at 1, however, there are more outliers beyond 3× CI and more variability in other elements. CI chondrite abundances used are those in Palme et al. 2014. See the main text for a description of uncertainties.

Figure A1.

Figure A1.

Open in a new tab

Elements ratioed to Si and to CI for Mg, Al, Si, Ca, Fe and Ni for a) 16 regions containing GEMS in anhydrous, chondritic IDP U2-17B19, b) 14 regions containing GEMS-like-objects in Febo-B-6, c) 10 regions containing GEMS-like-objects in Febo-B-9, d) 3 regions containing equilibrated aggregates and other non-GEMS aggregates in U2-17B19, e) 2 regions containing aggregates of minerals in Febo-B-6, and f) 1 region containing an aggregate of minerals in Febo-B-9. Solid black lines indicate the upper and lower boundaries for 3× CI. Error bars are smaller than the size of plotted symbols. Lines are included that connecting data points for each sample to guide the eye.

References

  1. Alesbrook LS, Wozniakiewicz PJ, Jones AE, Price MC, Ishii HA, Bradley JP and Brough N 2017 Atmospheric collection of extraterrestrial dust at the Halley VI Research Station, Antarctica (abstract #1805). 48th Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  2. Bradley JP, Brownlee DE, and Veblen DR 1983. Pyroxene whiskers and platelets in interplanetary dust: evidence of vapor phase growth. Nature 301:473–477. [Google Scholar]
  3. Bradley JP, Brownlee DE, and Fraundorf P 1984. Discovery of nuclear tracks in interplanetary dust. Science 226: 1432–1434. [DOI] [PubMed] [Google Scholar]
  4. Bradley JP and Brownlee DE 1986. Cometary particles: Thin sectioning and electron beam analysis. Science 231:1542–1544. [DOI] [PubMed] [Google Scholar]
  5. Bradley JP, Keller LP, Thomas KL, Vanderwood TB and Brownlee DE 1993 Carbon analysis of IDP’s sectioned in sulfur and supported on beryllium films. 29th Lunar and Planetary Science Conference, 173–174. [Google Scholar]
  6. Bradley JP 1994. Chemically anomalous, preaccretionally irradiated grains in interplanetary dust from comets. Science 265: 925–929. [DOI] [PubMed] [Google Scholar]
  7. Bradley JP, Wozniakiewicz P and Ishii HA 2011 Constraints on the cosmochemical significance of element/Si ratios and oxygen isotopic compositions of GEMS from IDPs collected in silicone oil (abstract #1320). 42nd Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  8. Bradley JP 2014. Early grains in the solar nebula—Interplanetary dust particles. Meteorites and Cosmochemical Processes, Treatise on Geochemistry, ed Holland HD, Turekian KK (Elsevier–Pergamon, Oxford: ), 2nd Ed, Vol 1, pp 287–308. [Google Scholar]
  9. Brennan S, Ishii HA, Luening K, Ignatyev K, Pianetta P and Bradley JP 2007 Panning for gold: A case study in evaluating the elemental composition of comet Wild 2 dust in aerogel (abstract #1776). 38th Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  10. Brownlee DE, Olszewsky E, and Wheelock M 1982 A working taxonomy of micrometeorites (abstract). 13th Lunar and Planetary Science Conference pp. 71–72. [Google Scholar]
  11. Brownlee DE 1985. Cosmic dust—collection and research. Annual Review of Earth and Planetary Sciences 13:147–173. [Google Scholar]
  12. Brownlee DE 2014. The Stardust mission: Analyzing samples from the edge of the solar system. Annual Review of Earth and Planetary Sciences 42:179–205. [Google Scholar]
  13. Brownlee DE, Joswiak DJ, Bradley JP, Matrajt G, and Wooden D 2005 Cooked GEMS – Insights into the hot origins of crystalline silicates in circumstellar disks and the cold origins of GEMS (abstract #2391). 36th Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  14. Brownlee DE, Tsou P, Aléon J, Alexander CM Oí’ D, Araki T,Bajt S, Baratta GA, Bastien R, Bland P, Bleuet P, Borg J, Bradley JP, Brearley A, Brenker F, Brennan S, Bridges JC, Browning ND, Brucato JR, Bullock E, Burchell MJ, Busemann H, Butterworth A, Chaussidon M, Cheuvront A, Chi M, Cintala MJ, Clark BC, Clemett SJ, Cody G, Colangeli L, Cooper G, Cordier P, Daghlian C, Dai Z, D’Hendecourt L, Djouadi Z, Dominguez G, Duxbury T, Dworkin JP, Ebel DS, Economou TE, Fakra S, Fairey SAJ, Fallon S, Ferrini G, Ferroir T, Fleckenstein H, Floss C, Flynn G, Franchi IA, Fries M, Gainsforth Z, Gallien J-P, Genge M, Gilles MK, Gillet P, Gilmour J, Glavin DP, Gounelle M, Grady MM, Graham GA, Grant PG, Green SF, Grossemy F, Grossman L, Grossman JN, Guan Y, Hagiya K, Harvey R, Heck P, Herzog GF, Hoppe P, Hürz F, Huth J, Hutcheon ID, Ignatyev K, Ishii H, Ito M, Jacob D, Jacobsen C, Jacobsen S, Jones S, Joswiak D, Jurewicz A, Kearsley AT, Keller LP, Khodja H, Kilcoyne ALD, Kissel J, Krot A, Langenhorst F, Lanzirotti A, Le L, Leshin LA, Leitner J, Lemelle L, Leroux H, Liu M-C, Luening K, Lyon I, MacPherson G, Marcus MA, Marhas K, Marty B, Matrajt G, McKeegan K, Meibom A, Mennella V, Messenger K, Messenger S, Mikouchi T, Mostefaoui S,Nakamura T, Nakano T, Newville M, Nittler LR, Ohnishi I, Ohsumi K, Okudaira K, Papanastassiou DA, Palma R, Palumbo ME, Pepin RO, Perkins D, Perronnet M, Pianetta P, Rao W, Rietmeijer FJM, Robert F, Rost D, Rotundi A, Ryan R, Sandford SA, Schwandt CS, See TH, Schlutter D, Sheffield-Parker J, Simionovici A, Simon S, Sitnitsky I, Snead CJ, Spencer MK, Stadermann FJ, Steele A, Stephan T, Stroud R, Susini J, Sutton SR, Suzuki Y, Taheri M, Taylor S, Teslich N, Tomeoka K, Tomioka N, Toppani A, Trigo-Rodríguez JM, Troadec D, Tsuchiyama A, Tuzzolino AJ, Tyliszczak T, Uesugi K, Velbel M, Vellenga J, Vicenzi E, Vincze L, Warren J, Weber I, Weisberg M, Westphal AJ, Wirick S, Wooden D, Wopenka B, Wozniakiewicz P, Wright I, Yabuta H, Yano H, Young ED, Zare RN, Zega T, Ziegler K, Zimmerman L, Zinner E. and Zolensky M 2006. Comet 81P/Wild 2 under a microscope. Science 314:1711–1716. [DOI] [PubMed] [Google Scholar]
  15. Busemann H, Nguyen AN, Cody GD, Hoppe P, Kilcoyne ALD, Stroud RM, Zega TJ and Nittler LR 2009. Ultra-primitive interplanetary dust particles from the comet 26P/Grigg-Skjellerup dust stream collection. Earth and Planetary Science Letters 288:44. [Google Scholar]
  16. Chi M, Ishii HA, Simon SB, Bradley JP, Dai ZR, Joswiak D, Browning ND and Matrajt G 2009. The origin of refractory minerals in Comet 81P/Wild 2. Geochimica et Cosmochimica Acta 73:7150–7161. [Google Scholar]
  17. Cliff G and Lorimer GW 1975. The quantitative analysis of thin specimens. Journal of Microscopy 103:203–207. [Google Scholar]
  18. Dartois E, Engrand C, Brunetto R, Duprat J, Pino T, Quirico E, Remusat L, Bardin N, Briani G, Mostefaoui S, Morinaud G, Crane B, Szwec N, Delauche L, Jamme F, Sandt Ch., Dumas P 2013. UltraCarbonaceous Antarctic micrometeorites, probing the Solar System beyond the nitrogen snow-line. Icarus 224:243–252. [Google Scholar]
  19. De Gregorio BT, Stroud RM, Nittler LR and Kilcoyne ALD 2017. Evidence for reduced, carbon-rich regions in the solar nebula from an unusual cometary dust particle. Astrophysical Journal 848:113. [Google Scholar]
  20. Floss C, Stadermann FJ, Bradley JP, Dai ZR, Bajt S, Graham G and Lea AS 2006. Identification of isotopically primitive interplanetary dust particles: A NanoSIMS isotopic imaging study. Geochimica et Cosmochimica Acta 70:2371–2399. [Google Scholar]
  21. Flynn GJ 1989. Atmospheric entry heating: A criterion to distinguish between asteroidal and cometary sources of interplanetary dust. Icarus 77:287–310. [Google Scholar]
  22. Frank DR, Zolensky ME and Le L. 2014. Olivine in terminal particles of Stardust aerogel tracks and analogous grains in chondrite matrix. Geochimica et Cosmochimica Acta 142:240–259. [Google Scholar]
  23. Gainsforth Z, Butterworth AL, Jilly-Rehak CE, Westphal AJ, Brownlee DE, Joswiak D, Ogliore RC, Zolensky ME, Bechtel HA, Ebel DS, Huss GR, Sandford SA and White AJ 2016 Possible GEMS and ultra-fine grained polyphase units in comet Wild 2 (abstract #2366). 47th Lunar and Planetary Science CD-ROM. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Haas B, Stroud RM and Floss C 2017. FIB/STEM investigation of four impact craters from the Stardust comet return mission foils. Microscopy and Microanalysis 23:2190. [Google Scholar]
  25. Ishii HA, Bradley JP, Dai ZR, Chi M, Kearsley AT, Burchell MJ, Browning ND and Molster F 2008. Comparison of Comet 81P/Wild 2 Dust with Interplanetary Dust from Comets. Science 319:447–450. [DOI] [PubMed] [Google Scholar]
  26. Ishii HA and Bradley JP 2015. Transmission electron microscopy advances reveal subtle comet dust differences. Meteoritics and Planetary Science 50, A181. [Google Scholar]
  27. Ishii HA and Bradley JP 2018 Pristine amorphous GEMS in anhydrous interplanetary dust particles are very underdense (abstract #2368). 49th Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  28. Ishii HA, Bradley JP, Bechtel HA, Brownlee DE, Bustillo KC, Ciston J, Cuzzi JN, Floss C, and Joswiak DJ 2018. Multiple generations of grain aggregation in different environments preceded solar system body formation. Proceedings of the National Academy of Sciences, published ahead of print June 11, 2018. doi: 10.1073/pnas.1720167115. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Ishii HA, Wozniakiewicz PJ, Bradley JP, Farley K and Martinsen M 2017 Extraterrestrial dust collection at Mauna Loa Observatory, Hawaii (abstract #1141). 48th Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  30. Joswiak DJ, Brownlee DE, Pepin RO and Schlutter DJ 2000 Characteristics of asteroidal and cometary IDPs obtained from stratospheric collections: Summary of measured He release temperatures, velocities and descriptive mineralogy (abstract #1500). 31st Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  31. Kearsley AT 2017. How laboratory hypervelocity impact experiments have helped us to understand comet dust samples: a brief review. Procedia Engineering 204:43–50. [Google Scholar]
  32. Kehm K, Flynn GJ, Sutton SR and Hohenberg CM 2002. Combined noble gas and trace element measurements on individual stratospheric interplanetary dust particles. Meteoritics and Planetary Science 37:1323–1335. [Google Scholar]
  33. Keller LP, Bajt S, Baratta GA, Borg J, Bradley JP, Brownlee DE, Busemann H, Brucato J, Burchell M, Colangeli L, d’Hendecourt L, Djouadi Z, Ferrini G, Flynn G, Franchi IA, Fries M, Grady MM, Graham GA, Grossemy F, Kearsley A, Matrajt G, Nakamura-Messenger K, Mennella V, Nittler L, Palumbo ME, Stadermann FJ, Tsou P, Rotundi A, Sandford SA, Snead C, Steele A, Wooden D and Zolensky M 2006. Infrared spectroscopy of comet Wild-2 samples returned by the Stardust mission. Science 314:1728–1731. [DOI] [PubMed] [Google Scholar]
  34. Leroux H and Jacob D 2013. Fine-grained material encased in microtracks of Stardust samples. Meteoritics and Planetary Science 48:1607–1617. [Google Scholar]
  35. Matrajt G and Brownlee DE 2006. Acrylic embedding of Stardust particles encased in aerogel. Meteoritics and Planetary Science 41:1715–1720. [Google Scholar]
  36. Matzel JEP, Ishii HA, Joswiak D, Hutcheon ID, Bradley JP, Brownlee D, Weber PK, Teslich N, Matrajt G, McKeegan KD, MacPherson GJ 2010. Constraints on the formation age of cometary material from the NASA Stardust mission. Science 328:483–486. [DOI] [PubMed] [Google Scholar]
  37. Maurette M, Olinger C, Christophe Michel-Levy M, Kurat G, Pourchet M, Brandstätter F and Bourot-Denise M 1991. A collection of diverse micrometeorites recovered from 100 tonnes of Antarctic blue ice. Nature 351: 44–47. [Google Scholar]
  38. McKeegan KD, Aléon J, Bradley J, Brownlee D, Busemann H, Butterworth A, Chaussidon M, Fallon S, Floss C, Gilmour J, Gounelle M, Graham G, Guan Y, Heck PR, Hoppe P, Hutcheon ID, Huth J, Ishii H, Ito M, Jacobsen SB, Kearsley A, Leshin LA, Liu M-C, Lyon I, Marhas K, Marty B, Matrajt G, Meibon A, Messenger S, Mostefaoui S, Mukhopadhyay S, Nakamura-Messenger K, Nittler L, Palma R, Pepin RO, Papanastassiou DA, Robert F, Schlutter D, Snead CJ, Staderman FJ, Stroud R, Tsou P, Westphal A, Young ED, Ziegler K, Zimmermann L and Zinner E 2006. Isotopic compositions of cometary matter returned by Stardust. Science 314:1724–1728. [DOI] [PubMed] [Google Scholar]
  39. Messenger S, Keller LP, Stadermann FJ, Walker RM and Zinner E 2003. Samples of stars beyond the Solar System: Silicate grains in interplanetary dust. Science 300:105–108. [DOI] [PubMed] [Google Scholar]
  40. Messenger S, Nakamura-Messenger K, Keller LP and Clemmett SJ 2015. Pristine stratospheric collection of interplanetary dust on an oil-free polyurethane foam substrate. Meteoritics and Planetary Science 50:1468–1485. [Google Scholar]
  41. Nakamura T, Noguchi T, Tsuchiyama A, Ushikubo T, Kita NT, Valley JW, Zolensky M.E. ME, Kakazu Y, Sakamoto K, Mashio E, Uesugi K, Nakano T 2008. Chondrulelike Objects in Short-Period Comet 81P/Wild 2. Science 321:1664–1667. [DOI] [PubMed] [Google Scholar]
  42. Noguchi T, Ohashi N, Tsujimoto S, Mitsunari T, Bradley JP, Nakamura T, Toh S, Stephan T, Iwata N and Imae N 2014. Cometary dust in Antarctic ice and snow: Past and present chondritic porous micrometeorites preserved on the Earth’s surface. Earth and Planetary Science Letters 410:1–11. [Google Scholar]
  43. Palme H, Lodders K and Jones A 2014. Solar System Abundances of the Elements. Planets, Asteroids, Comets and the Solar System, Volume 2 of Treatise on Geochemistry, ed Davis AM (Elsevier, Oxford; ), 2nd ed, pp 15–36. [Google Scholar]
  44. Ogliore RC, Huss GR, Nagashima K, Butterworth AL, Gainsforth Z, Stodolna J, Westphal AJ, Joswiak D and Tyliszczak T 2012. Incorporation of a late-forming chondrule into comet Wild 2. Astrophysical Journal Letters 745:L19 (5pp). [Google Scholar]
  45. Rietmeijer FJM. 1998. Interplanetary Dust Particles. In Planetary Materials (ed Papike JJ, Rev. Min. vol. 36, pp 2–1 – 2–95. [Google Scholar]
  46. Rietmeijer FJM 2009. Stardust glass: Indignous and modified comet Wild 2 particles. Meteoritics and Planetary Science 44: 1707–1715. [Google Scholar]
  47. Rietmeijer F 2016. At the interface of silica glass and compressed silica aerogel in Stardust track 10: Comet Wild 2 is not a goldmine. Meteoritics and Planetary Science 51: 574–583. [Google Scholar]
  48. Sandford SA and Walker RM 1985. Laboratory infrared transmission spectra of individual interplanetary dust particles from 2.5 to 25 microns. Astrophys. J 291:838–851. [Google Scholar]
  49. Schramm LS, Brownlee DE and Wheelock MM 1989. Major element composition of stratospheric micrometeorites. Meteoritics 24:99–112. [Google Scholar]
  50. Stodolna J, Gainsforth Z, Leroux H, Butterworth AL, Tyliszczak T, Jacob J, Westphal AJ 2013. Iron valence state of fine-grained material from the Jupiter family comet 81P Wild 2 – A coordinated TEM/STEM EDS/STXM study. Geochim. Cosmochim. Acta 122:1–16. [Google Scholar]
  51. Stodolna J, Gainsforth Z, Butterworth AL and Westphal AJ 2014. Characterization of preserved primitive fine-grained material from the Jupiter family comet 81P/Wild 2 – a new link between comets and CP-IDPs. Earth & Planetary Science Letters 388:367–373. [Google Scholar]
  52. Taylor S, Lever JH and Harvey RP 1998. Accretion rate of cosmic spherules measured at the South Pole. Nature 392:899–903. [DOI] [PubMed] [Google Scholar]
  53. Taylor S, Lever JH, Alexander CMO’D, Brownlee DE, Messenger S, Nittler LR, Stroud RM. Wozniekiewicz PJ. and Clemett SJ. 2017 Sampling interplanetary dust particles from Antarctic air (abstract #2024). 48th Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  54. Tsou P, Brownlee DE, Sandford SA, Hörz F and Zolensky ME 2003. Wild 2 and interstellar sample collection and Earth return. J. Geophys. Res 108 (E10) 8113, doi: 10.1029/2003JE002109. [DOI] [Google Scholar]
  55. Velbel MA and Harvey RP 2009. Along-track compositional and textural variation in extensively melted grains returned from comet 81P/Wild 2 by the Stardust mission: Implications for capture-melting process. Meteoritics and Planetary Science 44:1519–1540. [Google Scholar]
  56. Verniani F 1969. Structure and fragmentation of meteoroids. Space Science Rev. 10:230–261. [Google Scholar]
  57. Westphal AJ, Stroud RM, Bechtel HA, Brenker FE, Butterworth AL, Flynn GJ, Frank DR, Gainsforth Z, Hillier JK, Postberg F, Simionovici AS, Sterken VJ, Nittler LR, Allen C, Anderson D, Ansari A, Bajt S, Bastien RK, Bassim N, Bridges J, Brownlee DE, Burchell M, Burghammer M, Changela H, Cloetens P, Davis AM, Doll R, Floss C, Grün E, Heck PR, Hoppe P, Hudson B, Huth J, Kearsley A, King AJ, Lai B, Leitner J, Lemelle L, Leonard A, Leroux H, Lettieri R, Marchant W, Ogliore R, Jia Ong W, Price MC, Sandford SA, Sans Tresseras J-A, Schmitz S, Schoonjans T, Schreiber K, Silversmit G, Solé VA, Srama R, Stadermann F, Stephan T, Stodolna J, Sutton S, Trieloff M, Tsou P, Tyliszczak T, Vekemans B, Vincze L, Von Korff N, Wordsworth M, Zevin D, Zolensky ME and 30714 Stardust@home dusters. 2014. Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft. Science 345:786–791. [DOI] [PubMed] [Google Scholar]
  58. Westphal AJ, Bridges JC, Brownlee DE, Butterworth AL, De Gregorio BT, Dominguez G, Flynn GJ, Gainsforth Z, Ishii HA, Joswiak D, Nittler LR, Ogliore RC, Palma R, Pepin RO, Stephan T and Zolensky ME 2017. The future of Stardust science. Meteoritics & Planetary Sci. 52:1859–1898. [Google Scholar]
  59. Wooden DH, Ishii HA and Zolensky ME 2017. Cometary dust: the diversity of primitive refractory grains. Phil. Trans. A 375:20160260. doi: 10.1098/rsta.2016.0260. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Wozniakiewicz PJ, Bradley JP, Zolensky ME, Brownlee DE and Ishii HA 2011. Kwajalein Atoll: A new collection site for micrometeorites. Meteoritics & Planetary Science 46, A253. [Google Scholar]
  61. Wozniakiewicz PJ, Bradley JP, Price MC, Zolensky ME, Ishii HA, Brownlee DE, Dearborn D, Jones T, Barnett B, Yakuma S, Letendre T, Gonzalez C, Bastien R and Rodriguez M 2014 Initial results from the Kwajalein micrometeorite collections (abstact #1823). 45th Lunar and Planetary Science Conference CD-ROM. [Google Scholar]
  62. Wozniakiewicz PJ, Kearsley AT Burchell MJ, Price MC, Ishii HA. and Cole MJ. 2018. Preparation of large Stardust aluminum foil craters for analysis. Meteoritics & Planetary Science 53: 1066–1080. [Google Scholar]
  63. Zolensky ME, Zega TL, Yano H, Wirick S, WestphalA J , Weisberg MK, Weber I, Warren JL, Velbel MA, Tsuchiyama A, Tsou P, Toppani A, Tomioka N, Tomeoka K, Teslich N, Taheri M, Susini J, Stroud R, Stephan T, Stadermann FJ, Snead CJ, Simon SB, Simionovici A, See TH, Robert F, Rietmeijer FJM, Rao W, Perronnet MC, Papanastassiou DA, Okudaira K, Ohsumi K, Ohnishi I, Nakamura-Messenger K, Nakamura T, Mostefaoui S,Mikouchi T, Meibom A, Matrajt G, Marcus MA, Leroux H, Lemelle L, Le L, Lanzirotti A, Langenhorst F, Krot AN, Keller LP, Kearsley AT, Joswiak D, Jacob D, Ishii HA, Harvey R, Hagiya K, Grossman L, Grossman JN, Graham GA, Gounelle M, Gillet P, Genge MJ, Flynn G, Ferroir T, Fallon S, Ebel DS, Dai ZR, Cordier P, Clark B, Chi M, Butterworth AL, Brownlee DE, Bridges JC, Brennan S, Brearley A, Bradley JP, Bleuet P, Bland PA, Bastien R 2006. Mineralogy and petrology of comet 81P/Wild 2 nucleus samples. Science 314:1735–1739. [DOI] [PubMed] [Google Scholar]
  64. Zolensky M, Nakamura-Messenger K, Rietmeijer F, Leroux H, Mikouchi T, Ohsui K, Simon S, Grossman L, Stefan T, Weisberg M, Velbel M, Zega T, Stroud R, Tomeoka K, Ohnishi I, Tomioka N, Nakamura T, Matrajt G, Joswiak D, Brownlee D, Langenhorst F, Krot A, Kearsley A, Ishii H, Graham G, Dai ZR, Chi M, Bradley J, Hagiya K, Gounelle M, Keller L and Bridges J 2008. Comparing Wild 2 particles to chondrites and IDPs. Meteoritics and Planetary Science 43:261–272. [Google Scholar]

RESOURCES