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- Experimental Results
- Discussion and Conclusions
Abstract– We used a combination of different analytical techniques to study particle W7190-D12 using microinfrared spectroscopy, micro-Raman spectroscopy, and field emission scanning electron microscopy (FESEM) energy dispersive X-ray spectroscopy (EDS). The particle consists mainly of hematite (α-Fe2O3) with considerable variations in structural disorder. It further contains amorphous (Na,K)-bearing Ca,Al-silicate and organic carbon. Iron-bearing spherules (<150 nm in diameter) cover the surface of this particle. At local sites of structural disorder at the hematite surface, the hematite spheres were reduced to FeO in the presence of organic carbons forming FeO-spheres. However, metallic Fe spheres cannot be excluded based on the available data. To the best of our knowledge, this particle is the first detection of such spherules at the surface of a stratospheric dust particle. Although there is no definitive evidence for an extraterrestrial origin of particle W7190-D12, we suggest that it could be an IDP that had moved away from the asteroid-forming region of the early solar system into the outer solar system of the accreting Kuiper Belt objects. After it was released from a Jupiter family comet, this particle became part of the zodiacal cloud. Atmospheric entry flash-heating caused (1) the formation of microenvironments of reduced iron oxide when indigenous carbon materials reacted with hematite covering its surface resulting in the formation of FeO-spheres and (2) Na-loss from Na,Al-plagioclase. The particle of this study, and other similar particles on this collector, may represent a potentially new type of nonchondritic IDPs associated with Jupiter family comets, although an origin in the asteroid belt cannot be ignored.
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- Experimental Results
- Discussion and Conclusions
For almost four decades, dust particles have been collected in the stratosphere between 17 and 19 km altitudes on flat-plate collectors that are carried aloft underneath the wings of high-flying aircraft (Brownlee 1978, 1985; Mackinnon et al. 1982; Zolensky et al. 1994; Rietmeijer 1998). The collected particles include interplanetary dust particles (IDPs), up to a few to hundred microns in size, that survived burning up in the mesosphere. With no exceptions, all of the collected IDPs experienced flash-heating when decelerating in the mesosphere. Specific rapid, thermally induced, modifications to some of the chemical and/or mineral properties of these IDPs are recognizable, and they are indicators of peak-heating temperatures (Flynn 1994; Rietmeijer 1998, 2002). About fifteen individuals, approximately 10–15 μm IDPs in size, will be collectable during an approximately 40 h collection period using a 30 cm2 flat-plate collector (Brownlee 1978, 1979, 1985). The main driving force behind these collections was, and still is, to have multiple low-cost sampling missions of comet and/or asteroid dust. Particles with a chondritic (solar) composition for the major elements (Mg, Al, Si, S, Ca, Fe, and Ni) and an aggregate structure are considered remnants of the least modified solar system materials. The collected chondritic IDPs showed systematic mineralogical differences that led to an infrared spectroscope classification scheme (Sandford and Walker 1985) and later to the acquisition of IR-reflectance spectra of individual chondritic porous IDPs (Bradley et al. 1992). Large nonchondritic IDPs ranging from 10 μm to approximately 60 μm in size are frequently collected, but they are infrequently analyzed. They are mostly forsterite and Fe-Ni sulfide IDPs (Christoffersen and Buseck 1986; Zolensky 1987; Schramm et al. 1989; Steele 1990; Zolensky and Barrett 1994; Rietmeijer 1996). Nonchondritic fragments of the much larger chondritic aggregate IDPs resemble the nonchondritic IDP grain sizes. Some of these particles are non-CI fragments of a cluster IDP (Thomas et al. 1995). They include Na-bearing, high-silica, and plagioclase-like particles as inferred from the reported compositions (Rietmeijer 1998). The data for the IDP-size fragments of the cluster IDP L008#5 (Thomas et al. 1995) lack many important details such as information about their amorphous or crystalline structure and if they are mixtures of minerals and glass, but the chemical and mineralogical variations among these fragments are quite large. Table 1 summarizes the types of nonchondritic IDPs that have been found.
Table 1. Chondritic and nonchondritic IDPs, on average 10–15 μm in size, in the Earth’s lower stratosphere (modified after Rietmeijer 2002).
|Chondritic aggregate IDPs||Nonchondritic IDPs (Chondritic aggregate material often adheres to surface)|
|A matrix of nanometer-scale amorphous silicates with variable amounts of embedded approximately 5 μm MG,FE- and Ca,Mg,Fe-silicates, Ni-free and low-Ni pyrrhotite, iron oxides|| |
| ||(1) Silicate IDPs, mostly Mg,Fe-silicates, and Mg(Fe),Ca,Al-Silicates|
| ||(2) Sulfide IDPs, mostly NI-FREE and low-Ni pyrrhotite; rare pentlandite|
| ||(3) Refractory, Ca,Ti,Al-rich IDPs|
| ||(4) (Rare) plagioclase-like IDPs|
| ||(5) IDPs that are admixtures of variable amounts of IDPs types 1–4|
|Cluster IDPs are mixtures of variable amounts of aggregate IDPs and nonchondritic IDPs (1)–(5)|| |
Iron oxides and Fe-oxyhydroxides, viz. magnetite, maghémite, hematite, and goethite, were found in several IDPs (Fraundorf 1981; Fraundorf et al. 1981; Rietmeijer et al. 1999; Rietmeijer 2002). Rotundi et al. (2007) reported micro-IR and micro-Raman measurements for five IDPs with evidence for the presence of maghémite and/or magnetite, including a very porous aggregate with maghémite and hematite that also showed the infrared silicate bands of olivine and pyroxene. Several Fe-oxide rich fragments ranging from 5 × 7 μm to 10 × 15 μm are also part of cluster IDP L0008#5 (Thomas et al. 1995). Weak Raman bands attributed to hematite and pyroxene-like infrared absorption features were previously detected in IDP 3U2 (Fraundorf et al. 1982). The pyroxene assignment was confirmed by EDX analysis. Electron diffraction of this particle supported the presence of magnetite (Fe3O4) or maghémite (α-Fe2O3) mixed with olivine. The D and G Raman bands associated with amorphous carbon were also detected in this IDP (Fraundorf et al. 1982).
Although not listed in Table 1, chondritic aggregate and cluster IDPs contain a wide range of organic compounds. Indeed, the 3.4 μm infrared feature allows the determination of the CH2/CH3 ratio in aliphatics (Matrajt et al. 2005). The D and G Raman bands allow the determination of the aromatic domain size. Thus, it was found that carbon in IDPs was mostly present as amorphous carbon (a-C), or hydrogenated amorphous carbon (a-C:H) for particles with a significant 3.4 μm infrared feature (Muñoz Caro et al. 2006). Such material corresponds to poorly graphitized carbon with an aromatic domain size between 1.1 and 1.6 nm (approximately 20–40 rings) that are either linked by aliphatic chains with CH2/CH3 ratios varying from 2.8 to 5.5 in IDPs containing a-C:H, or a carbon sp3-skeleton in IDPs containing a-C amorphous carbon (Muñoz Caro et al. 2006).
Particles in the same size range as IDPs on dust collectors can also be to linked to terrestrial sources such as volcanic dust injected into the stratosphere. These particles have nonchondritic compositions and lack the telltale signs of flash-heating, for example, a (partial) magnetite and/or maghémite rim and/or disseminated crystals of these oxides. However, from the lack of such signs, an extraterrestrial source cannot be excluded. A case in point would be the hydrated, low-Ni, nonchondritic stratospheric dust particles (Rietmeijer 1992) that could be a new type of IDPs of lunar, Martian, or differentiated asteroid origin (Flynn and Sutton 1990, 1991).
We report here the results from combined micro-FTIR, micro-Raman, and FESEM-EDX analyses of the irregularly shaped, 10 × 9 μm, particle W7190-D12 collected over North America between July 17 and October 3, 1996 during accumulated period of 40 h. In Cosmic Dust Catalog volume 17, it is described as being opaque with a dull black luster. Those are properties commonly ascribed to stratospheric dust particles of “cosmic (C)” origin. From this catalog we quote, “particle type ‘Cosmic’ is used to conveniently group together all particles, which are judged to be of extraterrestrial origin, including those that have apparently experienced strong ablation heating or melting.” The smooth structures along the collected particle perimeter (Fig. 1) could be a macroscopic ablation feature. Particles with this particular morphology in other Cosmic Dust Catalogs were called chondritic rough IDPs (Rietmeijer and Warren 1994) that included IDPs with the distinctive petrological properties of hydrated CM-meteorite matrix (Bradley and Brownlee 1991; Rietmeijer 1996). On this stratospheric dust collector, 23 particles were identified as C-type particles, and 14 of them contained Si and Ca as the main element components, including particle W7190-D12. A majority of C-type IDPs listed in this particular catalog have an EDX spectrum that is qualitatively similar to that of particle W7190-D12. None of these particles are listed as possible fragments of cluster IDPs that might indicate an extraterrestrial origin. The high abundance of these particles on the collector in the lower stratosphere at the time of collection might suggest that these nonchondritic particles were part of a dust swarm, e.g., a dust cloud linked a bolide event (Klekociuk et al. 2005) or a volcanic dust cloud. However, the SEAN bulletins report no volcanic events during the time period between January and October 1, 1996, that ejected debris that could have reached the lower stratosphere across the North American airspace at the time of particle collection. Still, it is common to find volcanic ash in the stratosphere with no apparent recent volcanic eruption (Zolensky et al. 1989). However, this observation is limited to aerosol particles, i.e., sulfuric acid aerosol droplets, during the arctic winter period. It does not include approximately 10 μm sized solid dust particles. There is no record whether these particles were collected from a single catastrophic event or were gradually collected during the entire 40 h of exposure time. Although there is no definitive evidence for an extraterrestrial origin of these particles, we suggest that particle W7190-D12 and other similar particles on this collector were from the Zodiacal dust cloud and may be nonchondritic debris from a Jupiter family comet. If so, particle W7190-D12 represents another new type of nonchondritic IDPs.
Figure 1. Scanning electron microscopy image of particle W7190-D12. This image was performed at the Center of Astrobiology using a JEOL JSM-5600LV instrument. This low vacuum image was obtained using the backscattered electron detection mode at an acceleration potential of 20 kV and 10 mm working distance.
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Discussion and Conclusions
- Top of page
- Experimental Results
- Discussion and Conclusions
Based on its optical and chemical attributes, particle W7190-D12 was classified as a C-type particle (p. 67 of the particle catalog at http://curator.jsc.nasa.gov/dust/cdcat17/typesC.pdf). This is not a definitive classification. Especially in the case of a compact nonchondritic particle, additional data are required such as stable isotope analyses before a cosmic origin can be accepted. The provisional identifying criteria listed in the Cosmic Dust Catalogs make no allowance for Al- and/or Ca-rich particles such as (rare) refractory IDPs (Zolensky 1987), but stable oxygen isotopes of such a particle proved its extraterrestrial origin (McKeegan 1987). Nonchondritic IDPs can be compact mineral agglomerates and mono-mineralic particles often with fine-grained aggregate material that resembles the fine-grained matrix material of chondritic aggregate IDPs attached to the surface (Rietmeijer 2002). If such fine-grained aggregate matrix material had been attached to particle W7190-D12, it would have been melted during atmospheric entry forming the smooth material along its perimeter as was observed for sulfide IDPs (Rietmeijer 2004). No such material was present on this particle. Nor does this particle show a partial or complete Fe3O4 rim. In the absence of obvious flash-heating indicators (Rietmeijer 1998) and stable isotope data, proving an extraterrestrial origin for this particle will be challenging and in fact will ultimately rely on proof of flash-heating.
Consistency with an Extraterrestrial Origin
This study of combined micro-FTIR and micro-Raman spectroscopy, and FESEM-EDS of particle W7190-D12 that measures 10 μm × 9 μm shows an aggregate of a hematite grain partially embedded in amorphous Na,Ca-aluminosilica material and with probably similar grains also attached to its surface. The surface of this material is quite smooth. An organic carbon layer covers the particle surface that was analyzed in this study. It is undoubtedly a natural material.
This tabulation shows that particle W7190-D12 shares common properties with chondritic aggregate IDPs and comet Wild 2, but they are not proof of an extraterrestrial origin of the particle.
Indicators of Atmospheric Entry Flash-Heating
The smooth structures along the particle perimeter could be a macroscopic ablation feature, or at least serve as an indicator of particle modification. The still angular feature of the hematite grain suggests that, if the particle was heated, the temperatures stayed well below its melting point at 1665 °C. It also means that crystalline albite-anorthite would not have melted. Thus, we accept that the amorphous Na,Ca-aluminosilicate was always amorphous. Its original compositions are unknown, but the proposed Na-loss would be consistent with heating of particle W7190-D12. However, it does not offer tight thermal constraints, but Na-loss from these framework silicates is fast and does not require high temperatures. The Raman data show that hematite in the peripheral areas of the particle are more disordered than in the central part in the general area of analysis spot 1. We submit that thermal annealing might be the cause of this observed decrease in disorder. A thermal annealing study tracking the annealing of structural irregularities in nanometer scale hematite found that temperatures between 420 and 510 °C sufficed (Pérez-Maqueda et al. 2002). The presence of a 3.4 μm feature in the infrared spectrum of the particle indicates that the annealing temperature was less than 600 °C (Muñoz Caro et al. 2006). While these studies are not a direct analog to the conditions during atmospheric entry flash-heating of IDPs, it does suggest that the reduction in hematite disorder was probably a response to a heating event at low temperatures. In IDPs, most iron is present as ferrous iron, excluding GEMS that contain metallic iron (Bradley 1994). Flash-heating modifications during atmospheric entry in an oxidizing atmosphere causes oxidation and formation of ferric iron-bearing minerals, viz. magnetite and laihunite (Rietmeijer 1998). As the thermal spike during atmospheric entry is short-lived (5–15 s), diffusion distances need to be short, which is amply demonstrated by the nanometer-scale sizes of magnetite and maghémite grains in the rims on heated chondritic and nonchondritic IDPs alike. Size-wise, the <150 nm in diameter FeO-spherules in this particle could have formed during atmospheric entry. However, there are also indicators that flash-heating of C-bearing IDPs occurred in reducing environments (Rietmeijer 1998).
The particle W7190-D12 is exceptional among the collected natural stratospheric dust particles because it contains hematite with a surface layer of organic carbons. Surface heating could initiate a simple reaction of hematite reduction, viz. Fe2O3 + C = 2FeO + CO. In this reaction, the reagent was most likely organic carbon. The CO product of this reaction would most likely be desorbed from the particle and be lost as a gas phase molecule to the atmosphere. Although FeO spherules are found across the entire particle, there is a distinct concentration located at one side of the hematite grain (Fig. 6). This side shows the “micaceous” layering that would be favorable sites for spherule formation. We suggest that at such sites local structural disorder in hematite became the loci of Fe3+ reduction and the formation of FeO-spherules.
Particle W7190-D12 probably experienced mild thermal heating. When it is not a volcanic dust particle, the alternative is an extraterrestrial origin and heating occurred during deceleration in the Earth’s atmosphere. The FeO-spherules formed during atmospheric entry. They can be considered the equivalent of a partial iron-oxide rim on IDPs that decelerated in under-oxidizing conditions. Ablation of its organic materials created the reducing conditions to effectuate limited reduction of hematite in particle W7190-D12. Fe-Ni spherules on the surface of silicate spheres were observed in a few rare IDPs and appear to have been produced by reduction processes related to strong heating of organic materials (Brownlee et al. 2001). We have not proven the extraterrestrial origin for particle W7190-D12. No search for isotopic anomalies or solar flare tracks could be conducted because the particle was first crushed onto the infrared transparent window. Clearly, more work is needed on these particles to prove their currently putative extraterrestrial origin such as determinations of the noble gas contents or searching for solar flare tracks in other Ca-rich particles on the same collector. Also, experimental work is needed on the formation of FeO spherules in chondritic and nonchondritic IDPs with organic materials in the presence of ferric-iron minerals during simulated conditions of flash-heating.
The petrological properties of nonchondritic particle W7190-D12 are consistent with platy hematite, which is a common mineral in terrestrial iron ore occurrences. It was recently also identified as present at the Martian surface (Lane et al. 2002), which provides a potential extraterrestrial source for such particles assuming that there is an ejection mechanism to place them in Earth-crossing orbits. It is noteworthy that it has been suggested that nonchondritic IDPs, including hydrated low-Ni nonchondritic IDPs, could be a new type of interplanetary dust particle that could have a lunar, Martian, or differentiated-asteroid origin (Flynn and Sutton 1990, 1991; Rietmeijer 1992).
Assuming that the particle is a nonchondritic IPD, we might then speculate on a possible cometary origin. The mass of particle W7190-D12 is contained in hematite (ρ = 5.3 g.cm−3) and amorphous Na,Ca-aluminosilicate (ρ ≈ 2.7 g cm−3 for the mineral form, but less when amorphous estimated at ρ ≈ 2 g cm−3) in roughly equal proportions. The estimated density of this particle that is approximately 9 μm in diameter is then ρ ≈ 3.65 g cm−3. Love and Brownlee (1994) developed a matrix to estimate atmospheric entry peak heating temperatures for an “average” IDP (ρ = 2.0 g cm−3) striking the atmosphere at a 45° angle. An important invariable in this matrix is that the product of particle density and diameter is constant. Exploiting this constancy and using the diameter and estimated density for particle W7190-D12 its equivalent diameter is about 16 μm. Its corresponding peak heating temperature would be approximately 600 °C at an entry velocity of 10 km s−1 and approximately 1200 °C at an entry velocity of 20 km s−1 (i.e., a low-cometary velocity). There is no evidence that this particle was heated to approximately 1200 °C, but there is the possibility that this particle was heated to approximately 500 °C, which suggests that particle W7190-D12 is an IDP that entered the atmosphere at approximately 10 km s−1. The average atmospheric entry velocity of dust from the Zodiacal Cloud is 14.5 km s−1, but a sizeable fraction enters at velocities of approximately 12 km s−1. The estimated low entry velocity for this particle suggests that it was on an orbit of low eccentricity and low inclination to the ecliptic. Unfortunately, as its time of entry and direction (azimuth) of flight are unknown, there is no constraint on its source region. Interestingly, a recently developed dynamical model of the Zodiacal cloud showed that 85–95% of 100–200 μm particles in this cloud were ejected from Jupiter-family comets, <10% is contributed by particles from long-period comets, and that the contribution of asteroidal dust is no more than 10% (Nesvorný et al. 2010). Flynn (1996) had previously suggested that approximately 10 μm IDPs from the Kuiper Belt might be present among the Zodiacal dust cloud. We submit that particle W7190-D12 and other C-type IDPs on the same collector might be Jupiter-family comet debris. This particle W7190-D12 is the first detection of FeO spherules at the surface of a mildly heated nonchondritic IDP where it could form only in the presence of carbonaceous material. The present data cannot exclude the possibility that some fraction of the spherules could be metallic iron. The particle of this study may represent a potentially new type of nonchondritic IDP that could either be from the asteroid belt or was possibly associated with Jupiter-family comets.
Prior to the successful Stardust mission, this proposition would be without context. The results of this mission have shown that Kuiper Belt objects could harbor the same highly evolved minerals that were thought to be restricted to objects in the asteroid belt, such as those found in the unequilibrated ordinary chondrites (among others, Brownlee et al. 2006; Joswiak et al. 2009; Keller et al. 2006; Zolensky et al. 2006, 2008). This is an opportune time to search the NASA JSC Cosmic Dust Collection for nonchondritic IDPs that could be associated with Jupiter-family comets.
Acknowledgments–– We are grateful to M. E. Zolensky, G. J. Flynn, and D. E. Brownlee for very constructive reviews. We thank the curator and colleagues at the NASA JSC Curatorial Facility for providing particle W7190-D12. We acknowledge P. Dumas for assistance with the use of the micro-FTIR spectrometer, and G. Montagnac for the use of the micro-Raman spectrometer. We are grateful to E. Dartois for the Raman measurements. We thank G. Matrajt for requesting this particle. G. M. M. C. was supported by a Marie Curie Individual Fellowship from the European Union, a Ramón y Cajal research contract, and project AYA2008-06374 from the MCYT. F. J. M. R. was supported by NASA grant NNX07AM65G.