1. Introduction
Micrometeorites (MMs) are a subset of cosmic dust particles that compose most of the extraterrestrial material reaching Earth, making MMs integral in researching extraterrestrial chemistry and Earth’s geochemical supply of certain elements. These particles are significant factors in extraterrestrial chemistry in our solar system due to their abundance and omnipresence.
MM mineralogy and textures are a combination of parent body features mixed with phases formed by flash heating and quench cooling [
1]. Micrometeorites can reach up to 3 mm in diameter [
2] but are most commonly ~250 μm [
3]. Estimates of MM mass flux are subtle and varied. Suttle and Folco (2020) provided a recent summary of estimates based on some of the most prominent collections [
3]. Their estimate lies at 1555 ± 753 t/year, based on samples from the collection trap TAM65 of the 2017–2018 Italian Antarctic Campaign of the Programma Nazionale delle Ricerche [
3]. Most modern MM flux estimates fall within an order of magnitude of this [
4,
5,
6,
7,
8,
9]. These estimates have significant margins of error due to many factors, including weathering effects, inefficient collection methods, short accumulation windows, or small sample sizes. Genge et al. (2008) established the most current classification system for MMs [
1]. Firstly, MMs are identified as melted, partially melted, or unmelted [
1]. Relevant to this study are the melted MMs, classified as cosmic spherules (CSs) due to the spherical shape they obtain from atmospheric entry [
1]. Cosmic spherules can further be categorized as iron-rich (I-type), glassy with magnetite (G-type), or silicate (S-type), with S-type being by far the most abundant [
1]. Identifying petrographic features of CSs include significant fusion of pre-atmospheric entry phases and the presence of vesicles [
1]. There is also a broad range of common or identifying features among the CS subtypes, such as chondritic composition for S-types or a single large central void in I-types [
1]. Taylor et al. (2012) suggests that the majority of MMs come from fine-grained precursors with compositions similar to CI and CM meteorite classes [
7]. Silicates and sulfides present in CSs can be traced to remains from early planet formation [
10]. Micrometeorites of all classes can contain relict grains that have survived atmospheric entry unmelted [
11]. Examining the trace elements of relict olivines and pyroxenes can illuminate connections to extraterrestrial origin sources [
12,
13]. Relict grains are most commonly Mg-rich olivine (forsterite), followed by Fe-rich olivine and Mg-rich low-Ca pyroxenes [
11]. Pentlandite presence is associated with the sulfidization of iron and nickel [
14].
Genge et al. (2017) analyzed 500 urban MMs collected by amateur meteor collector Jon Larsen’s Project Stardust from roof gutters in Oslo, Norway [
15]. Of the 500 MMs collected, 48 were isolated for SEM and electron microprobe analysis and determined to be S-type cosmic spherules with identical chemistry, mineralogy, and texture to cosmic spherules collected in remote locations across Earth, such as Antarctica [
15]. These findings verified the hypothesis that MMs can be collected in urban environments. Larsen’s Project Stardust further continued, and it has provided an accessible introduction to micrometeorites for non-scientists [
16]. Larsen collected and identified anthropogenic contaminants (such as welding spherules and glass spherules from urban road dust) posing as MM and cosmic spherules, analyzing their chondritic chemistry and textures to distinguish extraterrestrial from terrestrial [
16]. Additionally, in the United States, citizen scientist, Scott Peterson, has collected urban MMs, verifying his findings through SEM analysis at the University of Minnesota [
17]. Similarly, scientists from the Museum für Naturkunde Berlin and the Freie Universität Berlin engaged the citizens of Berlin in a citizen science project, collecting MMs city-wide from rooftops [
18]. Kilograms of dust were sifted to visually isolate particles of interest before authentication of extraterrestrial origin with an electron microscope; over 60 MMs have been confirmed, furthering the validity of urban MM collection and providing a model for citizen science projects involving MMs [
18]. As a result, a misconception has formed in popular culture concerning the ease with which MMs can be collected in populated environments.
Comparable to what was explored by Genge et al. (2017) and Blake et al. (2018), this research utilized simple collection methods to gather particles in accessible locations rather than expeditions to polar regions or the deep sea [
15,
19]. Some recent studies have used synchrotron technology to investigate the properties of micrometeorites. Taylor et al. (2011) found that synchrotron computed microtomography was an effective non-destructive means to investigate sulfur in micrometeorites [
20]. Van Maldeghem et al. (2018) used synchrotron techniques X-ray fluorescence (XRF) and X-ray diffraction (XRD) to characterize micrometeorites collected in the transantarctic and on mountain summits [
21]. Dionnet et al. (2020) used synchrotron X-ray computed tomography (X-CT) to analyze giant MMs, allowing for non-destructive characterization based on the texture and visualization of porosity, which infers atmospheric entry [
22]. This study supports the viability of synchrotron techniques as an alternative to SEM analysis [
22]. This study uniquely examined suburban micrometeorites utilizing synchrotron technology and combined both research and educational objectives. We collected atmospheric particles locally in a simple manner and determined their origin by non-destructive synchrotron techniques. Unlike previous studies, we examined the origin of these locally collected atmospheric particles by non-destructive synchrotron techniques XRF, XRD, and X-ray absorption near-edge structure (XANES). In addition to gaining knowledge about MM collection, classification, chemistry, and their role in our solar system, students involved also gain hands-on experience collaborating in a team setting. Blake et al. (2018) cite this method of learning as project-based learning (PBL); learning occurs outside of the traditional classroom experience, instead focusing on collaborative approaches to conduct investigations and answer questions. PBL is accepted as a successful method to improve student learning [
19].
4. Discussion
Cosmic spherules are expected to have Ni-bearing iron metal, magnesium-rich olivine (forsterite), near chondritic chemical composition, heterogeneity due to relict grains, and Fe-bearing sulfides [
1,
7,
11]. Based on physical sample selection criteria, XRF, XANES, and XRD results, the tested particles were categorized by their chemical properties as cosmic spherules (
Table 2) or of terrestrial origin. The three cosmic spherules characterized in this research are round, magnetic, and approximately between 50 and 500 μm in diameter as described in Genge et al. (2008) and Noguchi et al. (2000) [
1,
24]. I-type (iron rich and magnetic) CS can contain Fe-Ni beads and are proven to be easily collected from modern sediments [
33,
34]. The absence of both nickel and copper was used as an indicator of terrestrial origin [
1], as the presence of chondritic Fe-Ni and sulfides strongly supports extraterrestrial origin [
15]. Particles determined to be likely extraterrestrial origin were found to contain forsterite; forsterite and pentlandite are recognized as extraterrestrial relict minerals [
11,
12,
13,
14,
35].
XRF qualitative data for iron and nickel in three of the collected particles, CS BayShore3, CS BayShore4, and CS WestIslipB, are consistent with previously reported data for cosmic spherules [
1,
7]. The 40-μm
2 imaging area (constrained by SRX instrumentation and limited beam availability) and the heterogeneity of MM samples prevent accurate identification of minor elemental ratios based on XRF data. The heterogeneity of metallic elements is presented in
Figure 5 and
Figure 6. For example, CS BayShore3 (
Figure 5b) shows a wide distribution of copper and zinc within the small area scanned, whereas iron is present in only part of this area, leading to an artificially high relative amount of copper and zinc, more than the smaller amounts typically found in meteorites (e.g., Mt. Egerton in
Figure 4 and others in
Table 1). Cosmic spherules show different mineral concentrations in grains when they are analyzed by scanning electron microscope (SEM) imaging and by energy dispersive X-ray spectroscopy (EDS) [
7,
11,
15].
Figure 6a shows the colocalization of iron and nickel within the meteorite Mt. Egerton, while
Figure 6b shows the separation of the same elements within BayShore3 supporting the CS heterogeneity. Taken together, XRF imaging and SEM imaging with EDS are complementary to each other.
Simple diagnostic analysis was applied to disqualify samples with terrestrial features. Zinc was detected within all samples, including in small amounts within the mounting material. A much larger quantity of zinc was detected in WestIslipA as compared to a relatively small iron signal; this overwhelming quantity, as well as a large quantity of titanium, indicated that the sample was not a micrometeorite. High titanium content in relation to iron is not consistent with what is found in MMs [
1]. Two samples from Sayville were determined to be terrestrial particles as they contained a large amount of titanium and no nickel, which suggests terrestrial origin (
Figure 5).
Sulfur k-edge XANES were measured in candidate particles to further differentiate between terrestrial and extraterrestrial particles. Hard X-ray XRF analysis at the SRX beamline, for example, of ESM2, showed a metallic composition consistent with MM. XRF maps at TES indicated high sulfur content; however, XANES showed that ESM2 had very little sulfur, indicating a high concentration of lead due to M-alpha fluorescence being misidentified as sulfur K-alpha fluorescence, since these signals overlap. The overwhelming presence of lead as opposed to sulfur is unlikely for an extraterrestrial source [
36]. The distinction between the use of hard and tender energy X-rays is highlighted in this example, as without the tender energy X-rays used for XANES at TES, this sample may have been misidentified as MM, showing the value in the use of multiple synchrotron analysis.
The heterogeneity of sulfur across the entire CS WestIslipB is shown in
Figure 8. The presence of sulfide is consistent with extraterrestrial origin, as the main constituent of sulfur in many types of meteorite is troilite, an iron sulfide [
37], and pentlandite, an iron nickel sulfide [
14]. Samples still showed the presence of some sulfate, but this can be attributed to the weathering effects of extended time submerged in water in the collection tubs and does not exclude potential extraterrestrial origin [
38]. The forms of sulfide existing within the meteorite standards suggest a difference in sulfide speciation between the meteorites and the micrometeorites. However, the presence of sulfide in any form in these samples is suggestive of extraterrestrial nature (
Figure 9) [
15,
20]. The sulfide species revealed in analyzed particles have yet to be exactly matched with species in sulfur databases; however, we interpret this as follows. The difference in sulfide phases between MMs and our compared meteorite standards is a result of the small size of MMs, which can lead to melting and mixing with Fe-Ni metal to result in minerals like pentlandite and troilite (
Figure 10) [
11,
12,
13,
14,
20,
35]. The intermediate and oxidized phases present in our MM samples can be ascribed to weathering processes after atmospheric entry [
38]. Though Sayville5 was characterized as terrestrial based on SRX analysis, XANES revealed minute sulfide within the sample, which requires further investigation. XRF mapping at TES shows heterogeneity of sulfur, silicon, and phosphorus content across CS BayShore3. XANES further revealed the CS BayShore3 sulfur content was in the form of sulfide (
Figure 9), which is consistent with the CS classification [
20] and sulfate, which may have been a result of weathering [
38]. Powder microdiffraction used for mineral identification revealed forsterite and pentlandite in CS BayShore3, which further supports its characterization as CS (
Figure 10) [
11,
12,
13,
14,
35]. Elemental distributions, speciation, and relative abundance data from SRX and TES in combination with XFM microdiffraction data show a clearer picture of extraterrestrial versus terrestrial nature of collected samples.
6. Conclusions
Characterizing the origin of particles collected on suburban rooftops as extraterrestrial requires more than one method of chemical and structural analysis. Synchrotron X-ray fluorescence imaging at different energies enables the observation and distribution of key elements in CS, including iron, nickel, and sulfur. X-ray absorption near-edge structure reveals the speciation of chemical elements, for example, sulfur was determined to be in the form of sulfide in extraterrestrial particles. Synchrotron X-ray microdiffraction was used to identify minerals; pentlandite and forsterite were identified in the CS. All together, these X-ray methods allow for the determination of elemental composition, speciation, and mineralogy of the CS. Synchrotron analysis gives us new insight into ways to analyze extraterrestrial materials. This also has advantages not available to SEM, such as non-destructive sample preparation, chemical speciation through XANES, and compositional sampling below the surface of particles. Synchrotron technology can complement traditional SEM as in Dionnet et al. (2020) [
22] and may provide the best outcomes for extraterrestrial identification.
Conducting authentic research from project inception through to the communication of results is impactful learning for high school students. They collaborated with scientists and teachers to design the experimental protocols and then used advanced tools at a scientific institution to collect and analyze data. Exposing students to cutting-edge research tools and real-world science motivates them to pursue careers in science, technology, engineering, and mathematics.