# Composition and Mineralogy of Nuclei Material of Short Period Comets Revealed by Recent Spacecraft Missions

Michael Zolensky, Cecile Engrand, Tomoki Nakamura, Denton Ebel

► To cite this version:

Michael Zolensky, Cecile Engrand, Tomoki Nakamura, Denton Ebel. Composition and Mineralogy of Nuclei Material of Short Period Comets Revealed by Recent Spacecraft Missions. Space Science Reviews, 2024, 220, pp.79. 10.1007/s11214-024-01111-z. http://dx.doi.org/10.1007/s11214-024-01111-z

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<figure><img src="image_1.png" /></figure>

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# Composition and Mineralogy of Nuclei Material of Short Period Comets Revealed by Recent Spacecraft Missions

Michael Zolensky¹ · Cecile Engrand² · Tomoki Nakamura³ · Denton Ebel⁴

Received: 3 April 2024 / Accepted: 10 September 2024

© The Author(s) 2024

## Abstract

We summarize mineralogical and compositional results from lab and in-situ spacecraft investigations of samples of comets 81P/Wild 2 and 67P/Churyumov-Gerasimenko which provide constraints to early solar system evolution. Wild 2 cometary solids were mineralogically similar to anhydrous chondritic Interplanetary Dust Particles with the exception that GEMS have not between reliably reported in the former. Important similarities include the very wide range of olivine and pyroxene bulk compositions (including minor elements), which probably reflects a wide range of sampled grain formation environments. The abundance of presolar materials in the Wild 2 samples is also approximately equal to that in anhydrous chondritic Interplanetary Dust Particles rather than being dramatically higher. High-temperature aggregates including chondrules (mainly type II), CAI and AOAs are also present in Wild 2, but probably at less than 10% overall total abundance. There was a pre-mission expectation that comet solids would be dominated by either non-crystalline materials, nanoscale crystalline materials, or presolar grains. None of these expectations proved to be correct. Organic matter was only well preserved in a few examined samples. These exhibit complex organic bonds, including mainly alkenes, aromatic C = C and carboxyl C = O, in a variety of textures including nanoglobules. The organic matter in Wild 2 samples show spectral similarities with that of insoluble organic matter (IOM) from carbonaceous chondrites. The chronology of the collected solids has not been well established, but there is no evidence for captured live ²⁶Al, which could indicate accretion after this nuclide had effectively completely decayed. The implications of these major results are that Wild 2 sampled widely across the entire solar nebula, much wider than any other known astromaterials, potentially at a relatively late time compared to the oldest available astromaterials. A final caution is that the captured Wild 2 materials described here possibly derived from a small number of interior regions of a single Jupiter Family comet. There is also limited mineralogical information from comet 67P provided by the Rosetta Mission. The investigated 67P dust particles also contained about 45 weight% of solid organic matter, and is similar to the IOM extracted from carbonaceous chondrites. The H/C ratio of 67P IOM is about twice that of most carbonaceous chondrite IOMs and about 5 times that measured in asteroid Ryugu whole-rock samples, but lower than the H/C ratio of volatile species measured in the 67P coma. This high H/C ratio in the coma species is in turn compatible with that of the soluble organic matter in investigated carbonaceous chondrites such as Murchison. The average N/C ratio of 67P dust particles is about one order of magnitude lower than the
solar photosphere value, but it is compatible with that of primitive carbonaceous chondrite meteorites and of Ryugu IOM.

# 1 Introduction

Thanks to the Stardust mission, we can now perform mineralogical and petrographic analyses of samples derived directly from the Jupiter-family comet 81P/Wild 2. The Stardust spacecraft captured comet coma dust particles close to Wild 2, relative velocity 6.2 km/sec, at 1.86 AU from the sun. A few definitions should be given here. A ‘particle’ is usually defined as a small object that behaves as a single unit in terms of its transport and physical properties. A ‘grain’, or ‘crystallite’ is a smaller domain of solid matter such as a single crystal. Thus, a particle can contain one or it can contain numerous grains. An ‘aggregate’ always contains multiple grains. The inner dust coma encountered the spacecraft at 235 km from the nucleus, so particles were captured within hours of their release from the nucleus. Dust collection was accomplished using 2 × 4 × 3 cm tiles of low-density silica aerogel, with a graded density from <0.01 g/cm³ at the surface to 0.05 g/cm³. The collector was returned to Earth on January 15, 2006. Wild 2 coma particles have been characterized with regards to chemical composition, mineralogy, crystallinity and crystal structure, and grain size. To varying degree all these characteristics have been significantly affected by the capture process at 6.2 km/sec into silica aerogel. Based on imaging and data from the spacecraft Whipple shield, the coma particles sampled by the Stardust Mission may have derived from just a few active jets, which would have sampled only a small volume of the interior of the comet nucleus (Brownlee et al. 2006). However, the available data are insufficient to definitely establish this possibility - we will just never know. Excellent earlier reviews of Wild 2 results are available (Hanner and Zolensky 2010; Brownlee et al. 2012; Brownlee 2014; Westphal et al. 2017).

The Rosetta mission studied comet 67P/Churyumov-Gerasimenko between August 2014 and September 2016. Three dust analyzers were onboard Rosetta that could characterize dust in the size range from ~50 nm to mm sizes. MIDAS (Micro-Imaging Dust Analysis System) was an atomic force microscope that could image the structure of the cometary dust with a resolution down to ~8 nm). GIADA (Grain Impact Analyser and Dust Accumulator) was a dust analyzer that detected several tens of grains with masses between ~10 µg to ~0.1 mg. COSIMA (Cometary Secondary Ion Mass Analyser) was a time-of-flight mass spectrometer, which performed analyses of dust collected on 1 cm² targets. The resolution of COSIMA was ~25 µm, providing the composition of cometary dust particles at the sub-mm scale.

# 2 Chemical Properties

## 2.1 Elemental Composition

The only direct measurement of the elemental composition of comet coma dust prior to the Stardust Mission came from impact-ionization mass-spectrometers (Kissel 1986) that encountered comet Halley on the Giotto and the two VEGA spacecraft in 1986. These measurements were within a factor of two of the CI meteorite abundances (Jessberger 1999), although the accuracy of these results is limited by being based on a total sample mass of less than a nanogram, and the fact that the spectrometers could never be calibrated because
<figure><img src="image_2.png" /><figcaption>**Fig. 1** Transmitted light images of tracks 32 and 69 and enlargements of 5 mineralogically diverse terminal particles. NASA JSC images</figcaption></figure>

of the particle sizes, their terrific impact speed and the complexities of impact ionization of actual comet particles.

The Wild 2 coma particles were disaggregated and partially melted and partially vaporized during capture in silica aerogel, producing tracks up to about 5 mm in length (Brownlee et al. 2006) (Fig. 1). The aerogel was produced by the Jet Propulsion Lab, and carried some carbon, Fe and other trace elements residual from the production and bakeout processing. This silica aerogel was the cleanest, and most transparent that was known at the time, and was this well suited to the role of capture medium. Many but by no means all of the captured particles appear to have all been aggregates, where the densest and strongest components of the particles were deposited as aggregates down to single grains along and at the ends of the tracks (Figs. 1 & 2). Weaker and more volatile components were deposited as melt and vapor deposits along the entire lengths of the tracks. Some tracks were apparently made by nearly single grains (Nakamura-Messenger et al. 2011). A good reference on the use of track shape to infer the makeup of the impacting particle is Kearsley et al. 2012. The bulk elemental compositions of entire particle “tracks” were measured by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and synchrotron X-ray Fluorescence (SXRF) for elements having atomic number ≥16 (Flynn et al. 2006). One of the investigated tracks was split open exposing the interior to analysis by ToF-SIMS, permitting measurement of lower-Z elements, particularly Mg and Al. Since Si and O are the major elements in silica aerogel collection media, neither element could be determined in the tracks. Major elements in comet particle impact residues in craters in aluminum foil (which surrounded the aerogel tray frame) were determined using Energy Dispersive X-ray analyses (SEM-EDX) and ToF-SIMS. The crater residues provided information on light elements, especially Mg and Si, that could not be determined in the SXRF analysis of tracks, with the exception of Al, which was the main component of the foil. Combining analytical results from the cometary material in the tracks and in the craters provided the most complete bulk chemical analysis available of the Wild 2 coma particles (Flynn et al. 2006). The bulk composition of 23 particle tracks were measured by SXRF, with a calculated total ~115 ng of coma material. The crater residue analyzed by SEM-EDX resulted from the impact of ~215 nanograms of
<figure><img src="image_3.png" /><figcaption>**Fig. 2** Bright-field TEM image showing an olivine and pyroxene in sample C2027,2,69,1,1. Individual olivine and pyroxene crystallites are surrounded by amorphous matrix which may or may have been produced during capture in the aerogel. Image after Zolensky et al. (2008)</figcaption></figure>

material. Taken together, ~300 nanograms of coma material were analyzed in the investigation. However, this was still two orders-of-magnitude more than analyzed at comet Halley. In addition, these Wild 2 samples are largely still available for additional (including repeat) measurements.

This task was complicated by the element heterogeneity since the impacting particles were aggregates of material having diverse compositions. Elements were normalized to Fe to permit comparison to meteorite compositions. For example, most Wild 2 particle element/Fe ratios for Track 19 (C2115,25,19,0,0) were observed to vary by two orders-of-magnitude along the track. Flynn et al. (2006) calculated whole track compositions by adding individual element abundances at specific points along the track to that of the single terminal particle. The calculated Fe-normalized whole track composition for Track 19 was consistent with CI bulk composition for S, Cr, Mn, and Ni, but enriched in the moderately-volatile elements Cu, Zn, Ga and Se, and depleted in Ca and Ge. Another problem was correctly applying X-ray absorption corrections. Captured Wild 2 particles tend to be “cocooned” by melted or compressed silica aerogel (see Nakamura-Messenger et al. 2011 for good examples of this phenomenon), complicating the correction for X-ray absorption, especially for lighter elements.

Flynn et al. (2006) ultimately calculated the Wild 2 coma grain mean composition by summing the measured abundance of each element over all 23 tracks. Most element/Fe ratios were observed to be consistent with the CI ratios at the 1-sigma confidence level. The exceptions are S, which was significantly depleted, and Cu, Zn, and Ga, which were enriched. These results for the Wild 2 particles are consistent with but significantly extend the Halley measurements. For example, the trace elements Cu, Zn, Ga, Ge and Se were measured for the first time.

The analyzed Wild 2 coma particles were enriched in the moderately-volatile elements Cu, Zn, and Ga compared to CI, which was also reported for anhydrous chondritic interplanetary dust particles (IDPs) (Flynn et al. 1996), some (possibly most) of which have a cometary origin (Brownlee et al. 1993). The CI meteorite element abundances are taken
to represent the Solar Nebula composition for non-volatile elements because of the good agreement between CI abundances and the composition of the Solar photosphere (Anders and Grevesse 1989; Lodders 2020, 2021), although not all elements in the solar photosphere are as precisely determined as they are in IDPs, CI chondrites and Wild 2 coma particles.

The dust particles of comet 67P/Churyumov-Gerasimenko (hereafter 67P) were mostly mm-sized and moving at rather slow speed (a few m/s) (Rotundi et al. 2015). The 67P particles were aggregates of grains with sizes down to ~30 nm (Mannel et al. 2019), and fragmented upon impact onto the collecting COSIMA targets (Langevin et al. 2016; Lasue et al. 2019).

COSIMA collected more than 35,000 particles and could analyze about 250 of them. The ROSINA gas phase mass spectrometer also measured the composition for elements like Na, Si, K, Ca and Fe of the surface of 67P cometary dust particles sputtered by the solar wind and of particles in the coma accidentally entering the ROSINA instrument. The compositions of these particles were chondritic for Na, enriched in K and Si compared to the chondritic value, and depleted in Ca (Wurz et al. 2015; Rubin et al. 2022). However, these results are complicated somewhat by the fact that while the refractories given in the Wurz et al. (2015) paper were compatible with sputtering by solar wind, the Rubin et al. (2022) data (except for Si) were collected during perihelion, when solar wind could not reach the surface of the comet. Accordingly, care must be taken when combining results from these two investigations.

The composition of ~30 particles for a larger set of elements could be deduced from the analyses of the COSIMA instrument. The elemental compositions of these 67P dust particles measured by COSIMA showed some variability, with the average composition found to be broadly chondritic with enrichments in C, N, O and Si compared to the chondritic value, and depletions in Mg and Ca (Fig. 1) (Bardyn et al. 2017; Sansberro et al. 2022). 67P dust particles contained a large proportion (about 45 weight% of high molecular weight solid organic matter, mixed with a mineral matrix, yielding a C/Si atomic ratio of 5.5 + 1.4/-1.2 (Bardyn et al. 2017; Fray et al. 2016). This atomic ratio is compatible with the photosphere value of 7.19 ± 0.83, and is about one order of magnitude larger than the CI value of 0.76 ± 0.10 (Lodders 2021). Regarding comet Wild 2, the carbon concentration could not be quantified in 81P/Wild 2 samples because of the high-speed collection of the samples in aerogel. A low C concentration was however observed, with very few preserved C-rich phases. This could have resulted from the high speed (and somewhat destructive) collection of the dust, or it could be that the dust collected by Stardust was not representative of the comet as a whole (Westphal et al. 2017).

The H/C and N/C ratios could be measured by COSIMA in 67P dust. The H/C ratio is 1.04 ± 0.16 (Isnard et al. 2019), and N/C = 0.035 ± 0.011 (Fray et al. 2017) (see also paragraph below “cometary organic matter”). Ammonium salts were recently discovered in the ROSINA data of 67P, and their presence at the surface of the comet is compatible with spectra taken with VIRTIS-M (Visible and Infrared Thermal Imaging Spectrometer, Mapping channel) (Altwegg et al. 2020; Poch et al. 2020; Altwegg et al. 2022). These ammonium salts (in particular ammonium hydrosulfides and fluorides), that were not measured in the 67P dust particles by COSIMA could account for the apparent missing nitrogen reservoir in the COSIMA data. In that case, these salts would have sublimated before analysis in COSIMA. Phosphorus and fluorine were also detected in the dust particles by COSIMA (Gardner et al. 2020).
**Fig. 3** Comparison of the D/H ratios of 5 Wild 2 coma dust particles (McKeegan et al. (2006) and 25 67P dust particles (Paquette et al. 2021), along with measured values for comet comae water and HCN, anhydrous chondritic IDPs, meteoritic insoluble organic material (IOM), and interstellar molecules. Figure is after McKeegan et al. (2006), which has the references to the other values

<figure><img src="image_4.png" /></figure>

## 2.2 Isotopic Compositions

The Stardust and Rosetta missions enabled the measurement of isotopic ratios on dust particles from comets Wild 2 and 67P, respectively. The hydrogen isotopic composition (D/H ratio) of Wild 2 samples varies from the terrestrial D/H value (1.5576 × 10⁻⁴) up to about three times this value in submicrometric zones (McKeegan et al. 2006; De Gregorio et al. 2010, 2011; Matrajt et al. 2008; Stadermann et al. 2008) (Fig. 3). The D/H ratio was measured in 25 dust particles from 67P by COSIMA, yielding a value of D/H = (1.57 ± 0.54) × 10⁻³ (Paquette et al. 2021). This value is about ten times larger than the terrestrial value. Such high D/H values have also been observed in organic molecules in cometary atmospheres (Meier et al. 1998; Müller et al. 2022).

The nitrogen isotopic composition of Wild 2 dust particles is not very anomalous, with values reaching about 1.5 times the terrestrial value in submicrometric regions (McKeegan et al. 2006). In most cases, there is no correlation between the nitrogen and hydrogen isotopic compositions of cometary dust particles.

The sulfur isotopic composition of comet Wild 2 samples is compatible with the reference value (Heck et al. 2012; Ogliore et al. 2012a,b), but a cosmic symplectite showed enrichment in ³³S (Nguyen et al. 2017). This composition could result from photochemical irradiation of solar nebular gas. The sulfur isotopic composition (³⁴S and ³²S) of 67P dust was measured by COSIMA, and is compatible with the reference value (Paquette et al. 2017). The ³³S isotope could not be quantified by COSIMA, due to mass interferences.

The oxygen isotopic composition of Wild 2 samples is compatible with that of carbonaceous chondrites and Ryugu samples for the ferromagnesian minerals, and as well as for the refractory minerals, which show ¹⁶O enrichments (Nakamura et al. 2008; Nakashima et al. 2012, 2023; Joswiak et al. 2017; Ogliore et al. 2015; Defouilloy et al. 2017; Zhang et al. 2023, 2024). The Δ¹⁷O composition of Wild 2 minerals is negatively correlated with their Mg content with O isotopes on the PCM line (see Zhang et al. 2024, for the most recent tabulation). This result supports formation of these particular grains in a very oxidized environment followed by outward migration to the comet accretion region. The COSIMA instrument could only measure the ¹⁸O/¹⁶O isotopic ratio in 67P dust particles (the ¹⁷O was not determinable due to mass interference with O¹⁶H) (Paquette et al. 2018). Given the large
error bars associated to the measured ratio (which is inherent to the analytical method) the measured values for 67P are compatible with the terrestrial value as well as the range of values measured in meteorites.

# 3 Mineralogy

## 3.1 Introduction

Most results to date have been obtained from only the coarser-grained portions of captured particles, which have been estimated to represent approximately 40% of the impacting comet coma grains on average (Flynn et al. 2006). However, this estimate was formed 20 years ago and should be reevaluated. In only a few cases thus far have we seen the true mineralogy of the finest-grained materials. Fortunately, in at least two recorded instances accumulations of the finest-grained, nano-scale Wild 2 materials firmly attached to coarser grains survived to the ends of tracks and are available for analysis to provide a window onto the nature of the finest-grained coma material. Also, fine-grained materials located in the upper portions of some tracks survived intact. Thus, our understanding of the finest-grained materials from Wild 2 is very incomplete and will only be revealed in full by future work. Readers should understand that the conclusions reached in this review largely pertain to the fraction of Wild 2 minerals coarser grained than 1 µm. Researchers have by now investigated approximately 225 separate particle capture tracks to varying degrees and have obtained a basic understanding of the these. However, practically every Wild 2 grain has possessed unique characteristics, so undoubtedly future work will reveal different materials and broaden the understanding of Wild 2 mineralogy. In addition, it appears that all captured grains derived from only two or three highly collimated jets originating from sites inside of the nucleus, which suggests that the mineralogical results could be unrepresentative of the nucleus as a whole. The mineralogy of examined Wild 2 coma grains is summarized in Table 1, and there compared to three other unequilibrated astromaterials that have not suffered significant aqueous alteration - namely CV3 and unequilibrated ordinary chondrites (UOCs), and anhydrous chondritic IDPs.

The captured particles are, in general, aggregates of submicron grains with a bulk chondritic composition, with most aggregates also containing one or more much larger individual crystals of (most commonly) olivine, pyroxene and Fe-Ni sulfides up to ~10 µm in size (Zolensky et al. 2006; Frank et al. 2014; Joswiak et al. 2017). Summarizing the results from the studied tracks, ~33% are dominated by olivine grains, ~24% by low-calcium pyroxene, ~10% by a fairly equal amount of olivine and pyroxene, and ~33% are dominated by other minerals, mainly Fe-Ni sulfides. A major result of analyses to date is that crystalline materials are abundant in Wild 2 coma grains, dominating the coarse-grained materials. Brief summaries of the critical minerals follow.

## 3.2 Olivine

Olivine and/or pyroxene are present in the majority of Wild 2 particles, with observed grain sizes ranging from submicrometer to over 10 µm. The olivine exhibits an unusually wide compositional range of Fo₃~100 (Fig. 4), with minor element abundances similar to the matrix olivine found in some unequilibrated carbonaceous chondrites and unequilibrated ordinary chondrites (Frank et al. 2014; Joswiak et al. 2012, 2017) (Fig. 5). The wide range of Fo content (Mg#) in Wild 2 is very similar to that observed in anhydrous chondritic IDPs
<table><caption>**Table 1** Mineralogy of Wild 2 Compared to Unequilibrated Meteorites and Anhydrous Chondritic IDPs</caption><thead><tr><th>Major phases</th><th>Wild 2</th><th>CV3<sup>1</sup></th><th>UOCs</th><th>Anhydrous IDPs</th></tr></thead><tbody><tr><td>Olivine</td><td>Fo<sub>5</sub>-100</td><td>Fo<sub>4</sub>-100<sup>2</sup></td><td>Fo<sub>45</sub>-100<sup>2</sup></td><td>Fo<sub>45</sub>-100<sup>3</sup></td></tr><tr><td>Low Ca Pyroxene</td><td>En<sub>52</sub>-98</td><td>En<sub>7</sub>-100<sup>4</sup></td><td>En<sub>7</sub>-100<sup>4</sup></td><td>En<sub>50</sub>-100<sup>5</sup></td></tr><tr><td>High Ca Pyroxene</td><td>Aug, Pig, Diop, Kos</td><td>Aug, Pig, Diop<sup>4</sup></td><td>Aug, Pig, Diop<sup>4</sup></td><td>Aug, Pig, Diop<sup>5</sup><br>Kos<sup>3</sup></td></tr><tr><td>Plagioclase</td><td>yes</td><td>yes<sup>4</sup></td><td>yes<sup>4</sup></td><td>yes<sup>5</sup></td></tr><tr><td>Phosphates</td><td>Apat</td><td>Apat<sup>4</sup></td><td>Apat, Merr<sup>4</sup></td><td>Apat<sup>5</sup></td></tr><tr><td>Sulfides</td><td>Troi, Pyrr, Pent, Spha<br>Cubanite (rare)</td><td>Troi, Pent<sup>4</sup></td><td>Troi, Pent<sup>4</sup></td><td>Troi, Pyrr, Pent,<br>Spha<sup>5</sup></td></tr><tr><td>Oxides</td><td>Mag (rare), Ilm, Chr,<br>Spinel</td><td>Mag, Ilm, Chr,<br>Spinel<sup>4</sup></td><td>Mag, Ilm, Chr,<br>Spinel, Rut<sup>4</sup></td><td>Mag, Ilm, Chr,<br>Spinel<sup>5</sup></td></tr><tr><td>Glass</td><td>?</td><td>yes<sup>4</sup></td><td>yes<sup>4</sup></td><td>yes<sup>5</sup></td></tr><tr><td>Fe-Ni Metal</td><td>?</td><td>yes<sup>4</sup></td><td>yes<sup>4</sup></td><td>yes<sup>5</sup></td></tr><tr><td>Carbonates</td><td>rare</td><td>rare<sup>4</sup></td><td>none noted</td><td>none noted</td></tr><tr><td>Crystalline Silica</td><td>Tri, Cri</td><td>none noted</td><td>none noted</td><td>none noted</td></tr><tr><td>Roedderite</td><td>rare</td><td>none noted</td><td>none noted</td><td>none noted</td></tr><tr><td>Schreibersite</td><td>rare</td><td>yes</td><td>rare</td><td>rare</td></tr><tr><td colspan="5"><strong>Multiphase Components</strong></td></tr><tr><td>Chondrules</td><td>yes</td><td>yes<sup>4</sup></td><td>yes<sup>4</sup></td><td>none noted<sup>5</sup></td></tr><tr><td>CAI</td><td>yes</td><td>yes<sup>4</sup></td><td>yes<sup>4</sup></td><td>Yes<sup>5</sup></td></tr><tr><td>AOAs</td><td>possibly</td><td>yes<sup>4</sup></td><td>yes<sup>4</sup></td><td>none noted</td></tr><tr><td>GEMS</td><td>?</td><td>none noted</td><td>none noted</td><td>very abundant<sup>5</sup></td></tr></tbody></table>

Abbreviations: IDPs: Interplanetary Dust Particles, UOCs: Unequilibrated Ordinary Chondrites, Aug: augite, Pig: pigeonite, Diop: diopside, Kos: kosmochlore, Apat: apatite, Merr: merrilite, Troi: troilite, Pyrr: pyrrhotite, Pent: pentlandite, Spa: sphalerite (or wurtzite), Mag: magnetite, Ilm: ilmenite, Chr: chromite, Tri: tridymite, Cri: cristobalite, CAI: Calcium-Aluminum-rich Inclusions, AOAs: amoeboid olivine aggregates, GEMS: glass with embedded metal and sulfides (called Granular- and Fundamental-Units by [5]).

(Zolensky and Barrett 1994; Zolensky 2022), although minor elements of IDP olivines have not been well measured. However, we note that the similarity of anhydrous chondritic IDP olivine major element compositions to that for Wild 2 suggests that a lack of a composition distrubution peak for forsterite (Fo<sub>97</sub>-100) is a general property of comets, not limited to Wild 2.

Wild 2 olivines include some with very elevated MnO, Al<sub>2</sub>O<sub>3</sub> and Cr<sub>2</sub>O<sub>3</sub> contents, up to 5.1, 0.85 and 0.95 wt%, respectively (Frank et al. 2014). About 25% of these Mn- and Cr-rich olivines contain ≪1% FeO. Olivines with enrichments in these elements have been reported in carbonaceous chondrites, micrometeorites, Ryugu and CI chondrite meteorites, and chondritic IDPs, though they are rare (Klöck et al. 1989; Rietmeijer 1998; Gounelle et al.

<footnote n="1">¹Includes both reduced and oxidized CV3s</footnote>

<footnote n="2">²Frank et al. (2014)</footnote>

<footnote n="3">³This paper, and included references</footnote>

<footnote n="4">⁴Brearley and Jones (1998)</footnote>

<footnote n="5">⁵Rietmeijer (1998)</footnote>

<footnote n="6">⁶Rubin and Ma (2021)</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 5** Comparison of the Cr₂O₃ vs FeO content of olivine from Wild 2 coma dust, anhydrous chondritic and refractory IDPs, Semarkona matrix and Krymka matrix. Wild 2 olivine composition is closest to mildly-metamorphosed Krymka matrix olivine. From Frank et al. (2014), Zolensky (2022)</figcaption></figure>

<figure><img src="image_6.png" /></figure>

2002; Frank et al. 2014; Zolensky 2022; Mikouchi et al. 2022) and indicate an unmelted source (Ebel et al. 2012). In general, the FeO-rich olivine in Wild 2 has a wide range of MnO/FeO ratios, similar to CR/CH isolated matrix grains, but also includes other MnO-rich grains that are rare in chondrites (Frank et al. 2014). On the other hand, the Cr₂O₃ content of the FeO-rich Wild 2 olivine is distinctly lower than isolated matrix grains in the least equilibrated carbonaceous- (e.g., Acfer 094 (ungrouped C2) and ALH A77307 (CO3.0)) and ordinary chondrites (e.g., Semarkona (LL3.00) and QUE 97008 (L3.05)). However, many Wild
2 olivines have a substantially higher Cr₂O₃ content than those in slightly metamorphosed Krymka (LL3.2). In fact, the overall distribution of Cr₂O₃ content in Wild 2 olivine appears to be similar to that for isolated matrix grains in MET 96503 (L3.10). This result suggests that mild thermal metamorphism equivalent to that experienced by type 3.1 ordinary chondrites has modified some of the Wild 2 olivine, which would be a maximum temperature of ~400 °C, with large uncertainties (Huss and Lewis 1994; Huss et al. 2006). Of course, since this putative thermal processing only affected a fraction of the Wild 2 olivine it, obviously occurred before final accretion of the comet.

According to Fukuda et al. (2020) Wild 2 olivine grains from track 77 exhibit only small variations in δ²⁵Mg values from -1.0 to 0.6‰), which can be explained by mass-dependent fractionation from Mg isotopic compositions of the Earth and bulk meteorites, suggesting that Wild 2 olivine grains formed in a chondritic reservoir with respect to Mg isotope compositions. They also reported that one ¹⁶O-rich Wild 2 LIME olivine grain in the same track shows a negative Mg isotope fractionation (δ²⁵Mg = -0.8 ± 0.4‰, δ²⁶Mg = -1.4 ± 0.9‰) relative to bulk chondrites. Minor element abundances of the olivine in this same grain are in good agreement with those of olivines from some carbonaceous chondrite AOAs. These observed similarities suggest that this particular grain formed in an environment similar to AOAs (Ebel et al. 2012), probably near the proto-Sun, and was then transported outward to the Kuiper belt where comet 81P/Wild 2 formed.

Unusual materials are constantly turning up among the Wild 2 coma particles. The olivine in the terminal particle of track 26 is nearly pure fayalite with up to 5.1 wt.% MnO and is the only Wild 2 olivine studied to date that has no known compositional analog in the meteorite record (Frank et al. 2014), although a single fayalite grain has been reported in an anhydrous chondritic IDP (Zolensky 2022). MnO-enriched olivine formed by an igneous process in the nebula would require very unusual, highly reducing conditions for Fe metal to condense, driving up the MnO/FeO ratio in the residual melt. This origin is incompatible with co-crystallization of fayalite and thus this grain must not have an igneous origin. This particle also contains tridymite (Zolensky et al. 2006; Joswiak et al. 2012). Equilibrium thermodynamic models by Zolotov et al. (2006) demonstrate that pure fayalite generally forms at lower temperatures, higher water/rock ratios, and at higher pressure than more MgO-rich olivine. These authors proposed a single process to explain the formation of fayalite/silica assemblages in chondrites, namely aqueous dissolution of amorphous silicate and kamacite, followed by precipitation of silica and fayalite. Amorphous silicates are expected to be present in comets (Bradley 1994; Wooden et al. 1999; Rietmeijer 2002; Lisse et al. 2006), and kamacite has been reported in the Wild 2 samples by Joswiak et al. (2012). In addition, this unusual Track 26 particle contains pentlandite and orthorhombic cubanite (ideally CuFe₂S₃), which can indicate low-temperature, hydrothermal processing (Berger et al. 2011). Thus, hydrothermal deposition of this assemblage appears to be a possible, if not the most likely, formation scenario. If this is correct, the extremely high MnO content in this particle's fayalite may be a compositional signature of a presumed precursor amorphous silicate unique to Wild 2 among known astromaterials. Of course, as described below, aqueous/hydrothermal activity in Wild 2 appears to have been very minor, localized, and/or heterogeneous. An alternative scenario is the collisional break-up of an early-forming KBO that underwent significant aqueous alteration. Material ejected from such KBO collisions could have been incorporated into Wild 2. Some evidence to support this may come from CI chondrites. For example, Gounelle et al. (2006) made trajectory calculations for the well-observed Orgueil (CI1) meteoroid fireball and inferred a probable KBO origin.

The major conclusions of the Frank et al. (2014) and Fukuda et al. (2020) studies of Wild 2 olivine were as follows: (1) Wild 2 accreted material from various carbonaceous
and ordinary chondrite-forming regions; (2) fragments from amoeboid olivine aggregates (AOAs), type I chondrules, and type II chondrules were incorporated into Wild 2; (3) there is a striking deficiency of material from type I chondrules in Wild 2 relative to unmetamorphosed chondrites; (4) many Wild 2 grains appear to have experienced secondary thermal metamorphism. Zhang et al. (2023) suggested that the Wild 2 olivines with very low $\Delta^{17}$O could all have formed as part of amoeboid olivine aggregates (AOA) which, like the CAI (see below), were transported outward from the inner disk of the solar system (Brownlee et al. 2006; Brownlee 2014); (5) relict grain compositions that are exceptionally rare in all chondrite chondrules (e.g. Marrocchi et al. 2019) appear to be quite abundant in Wild 2.

The latest, most precise, O isotope study of fragments (largely but not exclusively olivine) from a very large track (2 cm long), and consequently very large fragments, revealed a negative correlation between $\Delta^{17}$O and Mg#, and that oxygen isotopes were clearly on the PCM line, supporting their formation in a very oxidized environment followed by migration to the comet accretion region (Zhang et al. 2024). These authors proposed that the migrating grains were well pulverized and mixed before final incorporation into Wild 2.

Wild 2's extraordinarily diverse mineral assemblage indicates that its components were forged over a wider range of formation times and/or positions in the early solar nebula than any known single chondrite parent body. Frank et al. (2014) concluded that Wild 2 had a late assembly date and included material from the break-up and dispersion of diverse bodies, including some that had been internally heated. This is consistent with the apparent absence of extinct $^{26}$Al (Nakashima et al. 2015; Westphal et al. 2017) and generally longer accretion times for KBOs than for asteroids. Frank et al. further proposed that the olivine in the early solar system radial flow proposed by Brownlee et al. (2012) was dominated by $^{16}$O-rich, refractory and AOA-like forsteritic olivine grains, rare relict grain compositions, and type II chondrule fragments. Scarcity of type I chondrule olivine would yield the flat distribution of fayalite contents for Wild 2. Therefore, the radial transport of both $^{16}$O-rich forsterite and type II chondrule olivine was efficient, but transport of type I chondrule olivine was relatively inefficient. If $^{16}$O-rich olivine formed prior to typical type I chondrules, then the efficiency of outward transport of inner solar system material to the Kuiper Belt may have been episodic. These results and observations provide valuable constraints to dynamical astrophysical models.

## 3.3 Pyroxene

After olivine, pyroxene is the most abundant mineral group found among the Wild 2 samples. Diffraction studies revealed that a significant fraction of the magnesium-rich Wild 2 low-calcium pyroxenes are orthoenstatite, requiring slow cooling, but the majority of Wild 2 pyroxenes have unknown crystal structures (Leroux et al. 2008a,b,c). Thus, we don't know the ratio of ortho- to clinopyroxene, which limits interpretation of formation scenarios. The compositional range displayed by the low-calcium pyroxene is also as extensive as the olivine, with $En_{52-98}$ and a significant frequency peak centered at $En_{95}$ (Zolensky et al. 2006). Low-calcium pyroxene usually coexists with olivine in the Wild 2 particles, but the Mg/Fe ratios for coexisting olivine and pyroxene are not always similar. For example, Track 17 contains olivine in the range $Fo_{55-69}$, while associated low-calcium pyroxene is a far less homogeneous $En_{52-96}$. The extreme compositional range of Wild 2 low-Ca pyroxene is again similar to the anhydrous chondritic IDPs, and significantly broader than that observed in most chondrites, including Murchison (CM2) and Orgueil (CI1). It should be possible to measure compositional profiles across the boundaries of adjacent olivine and low-calcium pyroxene crystals and between olivine and metal grains, to determine the extent of Mg and Fe zoning, which would yield the thermal histories of these assemblages,
if they are in thermodynamic equilibrium. This is particularly important since the Wild 2 grains have apparently experienced such diverse formation histories. However, this project would require measurements of many grains to high compositional accuracy.

Fortunately, a few Wild 2 pyroxenes have been the subject of careful investigation. Both orthoenstatite and Ca-rich pyroxenes are frequently present, but the former predominates in abundance. Leroux et al. (2008a,b,c) reported pyroxene-bearing terminal particles which contain lamellar intergrowths of pigeonite and diopside on the (001) plane. This microstructure is typical for an igneous process and lamellae formation by exsolution during cooling. Width and wavelength of the lamellae indicate a cooling rate in the range 10-100 °C/h. This is within the range of experimentally-determined subsolidus cooling rates for chondrules (Schrader et al. 2018).

The abundance of Ca-rich pyroxene (mainly augite and diopside) in the Wild 2 particles appears to be beyond what is normally found in chondritic IDPs or unequilibrated chondrites (Leroux et al. 2008a,b,c; Joswiak et al. 2012). Some of the diopside could have formed during thermal metamorphism, but the available information is inconclusive. Joswiak et al. (2009, 2012) reported a high abundance of Na + Cr-rich pyroxene (kosmochlor) in half of the aerogel tracks they have investigated. These are part of an assemblage they named “Kool grains” (more properly called particles) (Kool = kosmochloric high-Ca pyroxene + FeO-rich olivine). The high Na, Cr and sometimes higher Mn contents of these pyroxenes appears to be in excess of what has been reported from most chondrites, although similar pyroxenes have been reported in the Orgueil CI1 chondrite (Greshake and Bischoff 1996) and in some anhydrous chondritic IDPs (Joswiak et al. 2009). Joswiak reported that fine-grained (submicron) Kool particles from Wild 2 have textures suggestive of crystallization from melts while coarse-grained (>1 µm) Kool particles are often glass-free and could have formed by thermal metamorphism in the nebula. The major and minor element distributions between clinopyroxenes and coexisting FeO-rich olivines in the Kool particles lacking glass were reported as being consistent with formation at or near equilibrium. However, in glass-bearing fine-grained Kool particles, high concentrations of Na in the clinopyroxenes were inconsistent with experimentally-determined partition coefficients at equilibrium. Finally, these authors proposed that Kool particles could have been precursors to type II chondrules, though evidence for this proposal from actual chondrule studies was apparently lacking.

Frank et al. (2013) extended their study of pyroxene compositions in Wild 2 and in the matrices of unequilibrated chondrites (CM2, CR2, CH3, CB3, CO3, CV3-oxidized, CV3-reduced, L/LL3.0-4, H3.15, EH3, EL6, Acfer 094 (ungrouped C2), and K3) and reported that these confirmed the results from their study of olivine (Frank et al. 2014) in all essential respects. They reported pyroxene compositions in Wild 2 characteristic of L, LL, CH, and aubrite matrix materials. They reported a preponderance of low-Fe, Mn-rich pyroxene in Wild 2, a composition that is extremely rare in chondrite matrix. The chemical compositions presented in their study provide clear evidence that in addition to diverse nebular components associated with multiple chondrite types, Wild 2 incorporated materials that likely derived from evolved, internally heated asteroids. This conclusion obviously requires late formation of the Wild 2 nucleus.

Bridges et al. (2012) reported that one terminal particle from track 154 consists of an Al-rich diopside (9-11 wt% Al₂O₃) and pigeonite (En₈₀Wo₁₅) with considerable TiO₂ (0.5 wt%) and Al₂O₃ (5.2 wt%), all consistent with its being a fragment of an Al-rich chondrule typical of carbonaceous chondrites. The oxygen isotopic composition of the particle is also consistent with derivation from an Al-rich chondrule, but distinct from a typical CAI.

To summarize, Wild 2 contains anhydrous ferromagnesian silicates either from a wide range of nebular formation locations, some of which were apparently not sampled by known
meteorites, or from a unique, previously unsampled, nebular reservoir. A significant number of these materials appear to have been affected by mild thermal metamorphism, with the environment (preaccretionary vs post-accretionary) being uncertain. Joswiak and coworkers have suggested that Kool pyroxenes formed through metamorphism at high pressure, which would require the rapid formation and early destruction of a large (500-1000 km diameter) parent body, an idea for which there is some support (e.g., Brown et al. 2007). However, Joswiak et al. (2012) point out that Kool pyroxenes could also have formed by condensation from a fractionated nebular gas reservoir which had been enriched in volatile components by partial evaporation of preexisting solids, or through magmatic processes at low pressures. Clearly, additional studies of Wild 2 pyroxene are desirable.

## 3.4 Sulfides

Sulfides are the only mineral group found in all extraterrestrial materials. Fe-Ni sulfides are also ubiquitous in the Wild 2 particles. Unfortunately sulfides were also one of the phases most affected by heating during capture in the aerogel (Barrett et al. 1992), making their study very difficult (Zolensky et al. 2006; Leroux et al. 2008a,b). Several coarse-grained Wild 2 particles are dominated by FeS or pentlandite. We refer here to troilite (stoichiometric FeS) and pyrrhotite ($Fe_{1-x}S$) collectively as “FeS” unless the exact phase was determined (e.g., Gainsforth et al. 2019). The vast majority of Wild 2 sulfides have compositions close to stoichiometric FeS, with less than 2 atom% Ni, although 7 atom% Ni has been reported (Gainsforth et al. 2019). The latter measurement is probably pentlandite exsolved within troilite or pyrrhotite. The general lack of compositions between FeS and pentlandite (with intermediate solid solution compositions) suggests (but does not require) that FeS and pentlandite formed directly as crystalline species, i.e., did not form as amorphous phases which later became annealed (Vaughan and Craig 1978). The remaining Wild 2 phases in the Fe-Ni-S ternary (approximately half analyzed so far) have compositions that trend from FeS directly towards the Fe apex of the Fe-Ni-S ternary diagram (e.g., Sack and Ebel 2006). These appear to be troilite with intermixed kamacite (Leroux et al. 2008a,b). These phases are almost certainly a result of capture heating in the aerogel, and their presence can be used to gauge the degree of capture modification of the associated Wild 2 grains.

The few verified pentlandite crystals in Wild 2 tracks are intriguing since this phase is frequently an indicator of low-temperature metamorphism under oxidizing conditions, and/or aqueous alteration (Zolensky and Thomas 1995; Bullock et al. 2005). Ni-rich sulfides are not found in totally anhydrous chondritic IDPs, but are found in IDPs that experienced aqueous alteration, and are abundant in carbonaceous chondrites that are aqueously altered. Therefore, at face value the presence of these few Ni-rich sulfides suggests that some Wild 2 particles could have experienced a limited degree of aqueous alteration, but does not require this. Gainsforth et al. (2019) described Ni-rich pyrrhotite exsolution in a large troilite crystal suggestive of heating to several hundred degrees C that was apparently not caused by sample capture.

Berger et al. (2011) reported a crystal of (Zn, Fe)S containing essentially equal amounts of zinc and iron in a Wild 2 particle. They called this phase sphalerite, although no diffraction data were provided to establish that it was not in fact the polytype wurtzite. The presence of true sphalerite in equilibrium with Fe and FeS (as was apparently the case here) could permit use of the sphalerite cosmobarometer (Hutchison and Scott 1983) to determine conditions of sulfide formation. Joswiak et al. (2012) also reported “sphalerite” in Wild 2 particles from two separate tracks, again solely on the basis of composition so again the phase could have been wurtzite. In this case the Zn sulfides were in contact with pyrrhotite, pentlandite and
ferroan olivine. Since the ZnS polytypes can have different stability relations, with wurtzite probably being the high-temperature form (see Bowles et al. 2011), verification of the exact phase present in Wild 2, an ostensibly simple task, could help constrain formation conditions.

Berger et al. (2011) found submicron blebs of cubanite in a small particle from track 26 that also contained pentlandite, fayalite and tridymite. As mentioned earlier, this fragment has thus far proved to be unique.

## 3.5 High-Temperature Rocks – Calcium Aluminum-Rich Inclusions, Amoeboid Olivine Aggregates and Chondrules

A major surprise of the Stardust Mission was the rather abundant occurrence (~0.5 vol%) of high temperature mineral assemblages, most notably calcium aluminum rich (CAI-like) inclusions (Zolensky et al. 2006; McKeegan et al. 2006; Simon et al. 2008; Chi et al. 2009; Schmitz et al. 2009; Joswiak et al. 2012, 2017; Rotundi et al. 2014), amoeboid olivine aggregate (AOA)-like objects, and aggregates with obvious igneous origins which are very similar to chondrules from carbonaceous chondrites (Nakamura et al. 2008). In a survey of the Wild 2 results, Joswiak et al. (2017) reported that approximately 2–3% of investigated particles larger than 1 µm are CAI-like and nearly 50% of all bulbous Stardust tracks contain at least one CAI. Accordingly, they estimated that ~0.5 volume % of captured Wild 2 coma dust is CAI-like.

The Wild 2 CAIs described to date basically have concentric mineral structures, with spinel-anorthite cores surrounded by Al, Ti-rich clinopyroxene (Fig. 6). The first and still best studied Wild 2 CAI -"Inti", from Track 25- consists mainly of anorthite, gehlenite, spinel and diopside. Crystals of osbornite (TiN), (V, Ti)N, and Fe-Ni metal are present within all phases except diopside. The metal grains contain inclusions of platinum-group element alloy and Mo-, W-alloys. All these phases have high formation temperatures, especially the nitrides, which would have condensed in the nebula at 1600-2000 K, depending upon the local oxygen fugacity (Ebel 2006). As reported by Chi et al. (2009), Track 25 fassaite (Ca(Mg, Ti, Al) (Si, Al)$_2$O$_6$) was found to have a Ti$^{3+}$/Ti$^{4+}$ ratio consistent with the fassaite in other solar system CAIs (Beckett 1986). The oxygen fugacity of formation estimated from this ratio (−19.4 ± 1.3), assuming equilibration among phases at ~1500 K, is near the canonical solar nebula value of −18.1 ± 0.3, and in close agreement with that reported for fassaite-bearing Allende CAIs (−19.8 ± 0.9).

It should be possible to measure compositional profiles across the boundaries of spinel within silicates to determine the extent of Mg zoning, which would elucidate one portion of the thermal history of the Wild 2 CAI. The oxygen isotope compositions of the Wild 2 CAIs are along the trend defined by carbonaceous chondrite CAIs (McKeegan et al. 2006; Simon et al. 2008). All of the cometary refractory objects are moderately refractory, fine grained, spinel-rich inclusions such as those found in primitive chondrites – none of the most refractory CAI types have been reported. This lack could indicate a higher abundance of small, moderately refractory CAI nodules in the nebula (Joswiak et al. 2017), but could also be misleading due to a still limited number of samples. The presence in Wild 2 of CAIs that are thought to have formed in the inner solar system was probably the first demonstration that there was transport of materials from the inner to the outer nebula, by processes still poorly understood (Brownlee et al. 2006).

Track 25 CAI osbornite and fassaite exhibit fine-scale variations in V and Ti, respectively (Chi et al. 2009). This lack of equilibration reveals that this CAI was never annealed at temperatures sufficient to cause chemical homogenization, and thus suggests relatively
<figure><img src="image_7.png" /><figcaption>**Fig. 6** CAI fragments recognized from among Wild 2 coma dust. (a) BSE image of a CAI fragment (called “Inti”) from track 25. (b) Dark field TEM image of a microtomed slice of a subsample from (a). (c) closeup of (b) showing details of the refractory mineralogy. (d) High-angle annular dark field image and (e) composite RGB element maps of a microtomed slice of CAI fragment WF216 from track 172, consisting of concentric layers of spinel, anorthite, and Al-poor to Al-rich diopside. A V-bearing osbornite inclusion in anorthite is indicated in (d). Os = V-bearing osbornite, Di = diopside, An = anorthite, Sp = spinel, Pv = perovskite, TPx = Ti-bearing pyroxene, Ge = gehlenite, K = kamacite. Images after Simon et al. (2008), Brownlee et al. (2012), Joswiak et al. (2017)</figcaption></figure>

early and rapid transport of this particular CAI from the inner to outer solar system. Chi et al. (2009) further found that Track 25 fassaite has a Ti³⁺/Ti⁴⁺ ratio requiring formation at reducing conditions (see above). This low oxygen fugacity appeared to rule out formation by the X-wind model, which features highly oxidizing conditions in a reconnection region in the inner solar nebular disk (Shu et al. 2001). Their preferred scenario was CAI formation of the most refractory CAI components (e.g. osbornite) within 0.5 AU of the protosun, followed by outward transport and later incorporation with the remaining CAI phases (including fassaite and perovskite). The final CAI then continued to travel outward to the comet-forming region beyond 15AU. Wild CAIs are most similar to fine-grained meteoritic CAIs (FGIs) (Joswiak et al. 2017). These are porous aggregates of similarly rimmed nodules that are usually only a micron or so across. Some FGIs in meteorites contain thousands of nodules which therefore must have been a dominant solid in some local regions in the solar nebula.

Numerous Wild 2 particle fragments have now been found to be identical in many or all respects to chondrules from carbonaceous chondrites (e.g. Nakamura et al. 2008; Leroux et al. 2008c; Zhang et al. 2023, 2024; Joswiak et al. 2024) as described in part above (Fig. 7). These chondrule-like Wild 2 grains are characterized by (1) igneous porphyritic or
<figure><img src="image_8.png" /><figcaption>**Fig. 7** Fragments of chondrules and a complete chondrule from among Wild 2 coma grains. (a) BSE image of a cross-section of track 35 fragment Torajiro, displaying a porphyritic texture. (b) BSE image of a cross section of a track 81 fragment (C2092,7,81,1,0) named "Pyxie", another porphyritic object; holes are from SIMS measurements; (c) BSE image of a cross section of a fragment from track 26 named "Ada", which contains crystals of tridymite with interstitial fayalite; (d) BSE image of a cross section of another fragment (C2054,0,35,4) from track 35 named "Lilly", which contains low-Ca pyroxene (Eng99Wo4) in contact with Fo91 olivine with 2.0 wt% MnO and Si- and Al-rich mesostasis; (e) Complete porphyritic chondrule from track 227 with olivine, chromite, kamacite and albitic glass mesostasis. OI = olivine, Mes = mesostasis, LPx = low-Ca pyroxene, HPx = high-Ca pyroxene, Sp = spinel, Chr = chromite, Pl = plagioclase, Tr = tridymite, K-kamacite, Fa = fayalite, SA = silica aerogel. Images after Nakamura et al. (2008), Joswiak et al. (2012), Nakashima et al. (2015), Gainsforth et al. (2015), Joswiak et al. (2024)</figcaption></figure>

poikilitic textures suggestive of partial melting at high temperatures; (2) presence of glass formed during rapid cooling, (3) dominant occurrence of Mg-rich olivine and low-Ca pyroxene with minor amounts of rounded Fe-metal inclusions suggestive of crystallization under reducing conditions, (4) the presence of ¹⁶O-rich “relict” olivine grains that have equilibrated Mg/Fe ratios because Fe (being smaller) could more easily diffuse into the “relict” grain while oxygen did not - the POP Wild 2 olivines are equilibrated in Fe but some retain large ¹⁶O effects, and (5) apparent absence of completely melted chondrules such as the barred olivine (BO) type. One notable chondrule-like object is a myrmekitic intergrowth of
nepheline, olivine, augite, plagioclase, and a Ca phosphate (Zolensky et al. 2008). Some of these objects contain crystalline silica-rich phases including tridymite, cristobalite and roederite, and other rare phases including schreibersite (Joswiak et al. 2012). Compared with typical porphyritic chondrules in chondrites, the crystal size of olivine in Wild 2 chondrule-like objects is much smaller – typically 2-10 µm. However, some chondrites, such as the ALH-85085 CH chondrite, have similarly fine-grained chondrules (Grossman et al. 1988). Major element abundances of olivine, pyroxene, and mesostasis glass in the chondrule-like particles are within the ranges of compositions defined by a majority of meteorite chondrules (Nakamura et al. 2008). However, the range of oxygen isotope ratios of these particles spans the entire range observed for chondrules in carbonaceous chondrites. None of the data plot on or above the terrestrial mass fractionation line, which clearly resolves these particles from ordinary, Rumuruti, and enstatite chondrite material (Nakamura et al. 2008). Therefore, the “chondrules” in Wild 2 are most similar to type I and II olivine-pyroxene chondrules in carbonaceous chondrites, which derive from C- and D-class asteroids (Tachibana et al. 2022), with type II being the most abundant. Therefore, Wild 2 chondrules appear to have a strong genetic relation with materials in the outer asteroid belt. The largest Wild 2 chondrule reported to date is 60 µm type II chondrule with Fo₆₂ olivine from Track 227 (Joswiak et al. 2024) (Fig. 7e).

It is worth noting that despite the fact that no barred chondrules fragments have been reported from Wild 2 samples, there is reason to look for them. Zhang et al. 2021, show two barred olivine chondrule fragments found in a single giant cluster anhydrous chondritic IDP that is thought (by them) to be cometary and shown to have mineralogies similar to Wild 2.

As shown by Nakashima et al. (2012) the $\Delta^{17}$O values vary inversely with Mg# for Wild 2 chondrule-like objects and carbonaceous chondrite chondrules, but not for chondrules from LL3 chondrites and enstatite type 3 chondrites. The $^{17}$O compositional range indicates that Wild 2 chondrule-like objects are most similar to chondrules in the CR and Tagish Lake-like chondrites (such as WIS91600), which indicates that they likely come from a reservoir with similar dust/ice ratios as these chondrites (Defouilloy et al. 2017). However, the differences in the Mg# range between olivine in Wild 2 and carbonaceous chondrite chondrules imply that the 81P/Wild 2 reservoir was comparatively more oxidized, with a higher dust enrichment.

Since the oxygen isotope compositions of these Wild 2 CAI-like and chondrule-like objects are identical to some meteorite analogues, these objects probably all formed within the inner solar system as opposed to some other disk environment. The $^{16}$O-richest reported composition of Wild 2 chondrule-like objects is $\sim -51\%_0 \delta^{18}$O (Defouilloy et al. 2017; Zhang et al. 2023) and that of the Wild 2 “CAI” Inti is $\sim -41\% \delta^{18}$O (Simon et al. 2008), suggesting that both sampled a common oxygen reservoir during formation. This is more $^{16}$O-rich than most bulk meteoritic CAIs, but very close to the compositions of the most $^{16}$O-rich, unaltered components of refractory inclusions from carbonaceous chondrites (Simon et al. 2008; Yurimoto et al. 2008). This implies that the Wild 2 chondrule-like objects and CAI formed in the same region, probably near the Sun. This in turn further supports Brownlee’s proposal that a solar system wide dynamic process was operating to carry inner solar system solids (CAI and chondrules) out to the comet nucleus formation region (Brownlee et al. 2006; Brownlee 2014).

As previously mentioned, some Wild 2 olivine grains are proposed to have originated within AOAs (Joswiak et al. 2017; Zhang et al. 2023), although these particular olivines reportedly exhibit a wide range of Mg# ($F_{o_{0-97}}$) with high $\Delta^{17}$O values largely between $\sim -2\%_0$ and $\sim 2\%_0$, suggesting formation in a very oxidized environment, which is unlike olivine from typical AOAs which are very $^{16}$O rich and nearly pure forsterite. Iron-poor
fragments have $\Delta^{17}\text{O}$ down to −6‰, and iron-rich ones tend to have higher $\Delta^{17}\text{O}$ (up to 7‰), displaying an overall negative correlation with their Mg#, which Zhang et al. (2023) suggest could be due to the addition or involvement of $^{16}\text{O}$-poor ice during their formation, as proposed for CR chondrules (Tenner et al. 2015). Zhang et al. (2023) further speculated that the Wild 2 chondrule- and AOA-like objects could have formed late (>3 Ma after CAI) in a localized region outboard from the CR chondrule-forming region and therefore migrated only a short distance to the Wild 2 nucleus accretion location.

## 3.6 Non-crystalline Silicates

Since it is commonly proposed that the earliest nebular silicates (at least) were dominated by poorly-crystalline materials (Paquette et al. 2011), it is fundamentally important to determine the nature of any amorphous cometary silicates that may be present in the Wild 2 samples. We know from infrared spectroscopy (as much as we can “know” anything from such observations) that glassy or amorphous silicates are present in comets (Hanner and Zolensky 2010). In the anhydrous chondritic IDPs of proposed cometary origin, submicron-sized partially amorphous aggregates called GEMS (glass with embedded metal and sulfides) can comprise over 50% of the volume (Ishii et al. 2008). Thus, it was anticipated that GEMS would be prevalent in the Wild 2 samples. Unfortunately, distinguishing small (0.1-0.5 µm) GEMS from the silica aerogel that melted during particle impact has proven to be extremely difficult. Leroux et al. (2008a) and Ishii (2019) reported on detailed studies of the material along a number of track walls, describing a silica-rich matrix where cometary material has mixed with melted aerogel. Fe-Ni and Fe-Ni-S inclusions are present, as are localized regions of high Mg abundance, along with minor amounts of Ca, Al, Fe. In contrast to the IDPs, the GEMS-like material in aerogel has very low Mg abundance. Moreover, this material is generally found along tracks having a terminal FeS particle. Similar material, including reduced Fe-Ni inclusions, has been produced in laboratory experiments when FeS particles were accelerated into silica aerogel (Barrett et al. 1992), leading to the conclusion that most, if not all, of the GEMS-like material among the Wild 2 samples is a byproduct of the impact process. On the other hand, nanometer scale compositional mapping has identified Mg-rich domains within the molten aerogel, of a size and composition consistent with GEMS (Keller and Messenger 2008; Leroux et al. 2008a). Thus, there are hints that cometary GEMS are present in the sample. Hopefully this rather important issue will be resolved in the near future.

Some chondrule-like objects contain unaltered mesostasis glass. For example, the track 35 terminal grain named “Torajiro” (C2054,0,35,6) contains glass (see composition in Nakamura et al. 2008). The complete porphyritic chondrule from track 227 includes albitic glass mesostasis (Joswiak et al. 2024).

## 3.7 Fine-Grained Material

As mentioned above, the majority of fine-grained comet coma materials were apparently melted and intimately mixed with silica aerogel during sample capture, making it very challenging to characterize. Fortunately, several investigated Wild 2 terminal particles include well-preserved fine-grained material, including “Febo” (track 57), “Ada” (track 26), “Iris” (track 74), “Andromeda” (track 191), and track 227 (Brownlee et al. 2006; Matrajt et al. 2008; Stodolna et al. 2012b; Ishii et al. 2018; Gainsforth et al. 2019; Joswiak et al. 2024). In each case a large crystal, usually a sulfide and/or ferromagnesian silicate, and one case a large chondrule (track 227) shielded adjacent nanoscale material from significant heating
<figure><img src="image_9.png" /><figcaption>**Fig. 8** Wild 2 particles exhibiting preserved fine-grained cometary material. (a) Bright field TEM image of a microtomed slice of a fragment from track 191 called “Andromeda”, showing the sulfide (chattered from microtomy) on the left, and fine-grained material on the right. (b) Composite RGB element maps of a reduced area from (a); red is troilite, light blue is probably olivine and/or low-Ca pyroxene, dark blue are nanoscale aggregates of silicates. (c) Dark field TEM image of an enstatite whisker (arrowed) from a fragment from track 74 called “Iris”, with an inset diffraction pattern showing that the elongation direction is roughly along the [100] direction. (d) TEM image of a microtomed slice of a terminal particle from track 57 called “Febo”. This is a zero-loss image obtained with the zero-loss peak of the area of the particle with the highest carbon abundance. (e) C-EELS map of (d), with the white areas indicated by the square and circle exhibiting the highest carbon abundance. The doughnut shaped C material in the circle may be a hydrocarbon nanoglobule. Images after Matrajt et al. (2008), Stodolna et al. (2012b), Gainsforth et al. (2019)</figcaption></figure>

during collection in aerogel (Fig. 8). The fine-grained material has been reported to include olivine, low-Ca pyroxene, kosmochloric diopside (Na-Cr coupled substitution), plagioclase-like material, Fe-Ni sulfides, Fe-Ni metal, probable chromite, poorly crystalline aluminosilicates, and C-bearing phases. All these phases are also found as coarser-grained material in the Wild 2 particles, with the possible exception of the carbonaceous material. The carbonaceous phases are systematically associated with <sup>15</sup>N enrichments, consistent with an organic, cometary material. In the particle Ada the carbonaceous phase is also associated with a D enrichment whose value is comparable to enrichments observed in IDPs (Matrajt et al. 2008). The C-rich areas are sometimes associated with O and show the presence of a carbonyl (C = O) feature at 288.5 eV, which also supports their organic nature. Although the size and level of the <sup>15</sup>N enrichment found in these C-rich areas is comparable with C-rich inclusions previously reported in IDPs, the C-XANES spectra of the organic materials in Febo and Ada is different from the ones observed in IDPs and carbonaceous chondrites.
Gainsforth et al. (2019) reported that the fine-grained material in “Andromeda”, which appears to be the best-preserved known example, does not have a solar composition. It is depleted in Na, Mg, Al, Ca, Cr, Mn, and Ni, and enriched in P, S, Fe, and K relative to CI. Clearly additional, similar investigations are required as additional samples of the fine-grained materials are found and investigated.

Carbon Electron Energy Loss Spectroscopy (EELS) of fine-grained assemblages in “Ada” and “Febo” reveal round structures that in form and basic chemistry appear identical to the organic nanoglobules now known to be present in most organic-bearing chondrites (Nakamura-Messenger et al. 2006; Matrajt et al. 2008; De Gregorio et al. 2013) and recently reported from asteroids Ryugu (Stroud et al. 2024) and Bennu (Nguyen et al. 2024). Matrajt et al. (2008) noted that these materials all exhibit <sup>15</sup>N enrichments up to 200‰ and D enrichments up to 500‰, requiring a low temperature formation location, i.e., outer solar system or possibly an interstellar molecular cloud.

The amount of presolar material present in Wild 2 has been reported to be consistent with anhydrous chondritic IDPs, and other primitive bodies (Floss et al. 2013). However, this estimate again suffers from the fact of preferential destruction of fine-grained materials during sample capture. The foils, therefore, play a major role in the estimate of presolar grain abundance in Wild 2, which remains, at best, a lower limit.

## 3.8 Low-Temperature Inorganic Phases

No phyllosilicates or hydrous phases have been definitively identified in any Wild 2 sample (Zolensky et al. 2006; Westphal et al. 2017). Despite the fact that significant heating and structural modification accompanied collection of many grains in the aerogel, we should have seen characteristic compositions, grain morphologies, and lattice fringes of phyllosilicates had they been present, even if they had been thermally modified (Barrett et al. 1992; Hörz et al. 2000). During heating, phyllosilicates lose interlayer water and OH, and interlayer lattice fringes collapse in a characteristic manner. Eventually, a neoformed anhydrous silicate is formed, generally orthopyroxene or olivine. Fortunately, the neoformed silicates generally retain the flaky morphologies of the original phyllosilicates, however, and so are still recognizable as transformed phyllosilicates. We have observed this phenomenon in naturally- as well as artificially-heated (metamorphosed) carbonaceous chondrites and analogues (Akai 1988; Zolensky et al. 1991; Barrett et al. 1992; Okudaira et al. 2004; Tonui et al. 2014). Additional evidence that some phyllosilicates would have survived capture is presented by the partial survival of scattered, delicate, low-temperature and organic-rich Wild 2 phases exhibiting deuterium and <sup>15</sup>N excesses (Sandford et al. 2006; Cody et al. 2008). These fine-grained materials survived capture in the aerogel with minimal heating when attached to larger, more refractory grains. Phyllosilicates would have behaved similarly.

Berger et al. (2011) reported a tiny amount of cubanite in track 26. On Earth cubanite is generally indicative of low-temperature, hydrothermal processing in terrestrial environments. However, this cubanite has not been reported in any additional Wild 2 sample. Another possible alteration product is the magnetite that has been reported in a few tracks (Hicks et al. 2017). Once again, magnetite appears to be rare in Wild 2 samples, unlike the situation in thoroughly aqueously-altered chondrite meteorites, Ryugu and Bennu samples (Nakamura et al. 2022; King et al. 2024), and in any case can be formed by igneous processes (Keller et al. 2022).

Micron-sized crystals of calcite and Fe-Mg carbonate have been reported from three Wild 2 particles (Mikouchi et al. 2007; Wirick et al. 2007), but it is now clear that carbonates
are exceedingly rare, if not mainly terrestrial contaminant grains. However, carbonates in metamorphosed carbonaceous chondrites have devolatilized to hydroxides or oxides (Tonui et al. 2014). Thus, it is possible that carbonates were more abundant in the Wild 2 particles before capture and capture heating, but this has not been established.

Thus, the lack of phyllosilicates, and rarity of carbonates among the examined Wild 2 grains suggests that they could not have composed more than a few percent of the more coarse-grained fraction of captured Wild 2 sample. Accordingly, Wild 2 seems to have not experienced any significant degree of aqueous alteration, or hosted liquid water at any time.

# 4 Organic Matter

One characteristic of comets is their high abundance of organics, which is already seen by remote sensing analyses in cometary atmospheres (Biver and Bockelée-Morvan 2019). These species are associated with sublimating ices when the comet gets closer to the sun, that release volatile organic compounds. These volatile species contain mainly CH(N)O bearing molecules, with a great variety of both saturated and unsaturated species containing CH-, CHN-, CHS-, CHO2-, CHNO- functional groups, and the CN radical and related molecules (Altwegg et al. 2017; Hänni et al. 2021, 2022). The aminoacid glycine, as well as methylamine and ethylamine were also found in the gas phase, originating probably preferentially from the sublimation of the dust particles rather than from ices from the nucleus (Altwegg et al. 2016; Hadraoui et al. 2019, 2021). Glycine was also found on Al foil exposed to the atmosphere of comet 81P/Wild 2 during the Stardust mission (Elsila et al. 2009). The ammonium salts stated earlier could be in the form of ammonium hydrosulphide and fluoride) (Altwegg et al. 2020, 2022; Poch et al. 2020). Organic matter was not very well preserved in the comet Wild2 samples brought back by Stardust, because of high-speed collection (Brownlee 2014; Keller and Messenger 2008; Sandford et al. 2006). In some place however, there were organic phases showing complex organic bonds, including mainly alkenes, aromatic C = C and carboxyl C = O, in a variety of textures, including organic nanoglobules (De Gregorio et al. 2011). The organic matter in Wild 2 samples show spectral similarities with that of IOM from carbonaceous chondrites (De Gregorio et al. 2011).

COSIMA/Rosetta analyses also showed that 67P dust particles contain about 45 weight% of solid organic matter, bearing similarities with the insoluble organic matter extracted from carbonaceous chondrites (IOM) (Bardyn et al. 2017; Fray et al. 2016). The atomic H/C and N/C ratios of 67P dust measured in the organic matter (1.04 ± 0.16 and 0.035 ± 0.011, respectively (Isnard et al. 2019; Fray et al. 2017)) can be compared to that of insoluble organic matter extracted from carbonaceous chondrites and from the analyses of asteroid Ryugu. The H/C ratio is about twice that of carbonaceous chondrite IOMs (Alexander et al. 2007), and about 5 times that measured in asteroid Ryugu whole-rock samples (Yokoyama et al. 2023), but lower than the H/C ratio of volatile species measured in the coma by ROSINA on Rosetta (H/C = 1.56 ± 0.04 (Hänni et al. 2022). This high H/C ratio in the coma species is in turn compatible with that of the soluble organic matter in the Murchison carbonaceous chondrite (Schmitt-Kopplin et al. 2010). The average N/C ratio of 67P dust particles is about one order of magnitude lower than the photosphere value (N/C = 0.24 ± 0.06) (Lodders 2021), but it is compatible with that of primitive carbonaceous chondrite meteorites (N/C ~ 0.03) (Alexander et al. 2007) and of Ryugu IOM samples (N/C ~ 0.15) (Yabuta et al. 2023).

## 4.1 Combined Textural and Compositional Study

Each stardust track represents a unique hypervelocity impact event. Tracks were cut from aerogel tiles in thin, triangular “keystones” (Westphal et al. 2002) translucent to optical
light. The nature of each track-forming event, including the original state of the impactor, is recorded in 3D track morphology. Material distribution has been recovered by synchrotron X-ray fluorescence (S-XRF) mapping of whole tracks (e.g., Flynn et al. 2006).

Ebel et al. (2009) used laser scanning confocal microscopy (LSCM), to acquire 3D im-
agery of whole tracks at high resolution (<80 nm/pixel in x and y, ~300 in z the optic
axis). They developed 3D deconvolution methods to perform first-order corrections of op-
tical aberrations induced by light diffraction and refractive index mismatches (Greenberg
and Ebel 2010; White et al. 2012). S-XRF mapping of 3D-imaged tracks was done at 2 mi-
crons/pixel. For many tracks, the S-XRF maps were done at two angles, allowing elimination
of aerogel surface contamination.

Greenberg and Ebel (2012) used these combined data to measure track volumes, entry hole size, and cross-sectional profiles of tracks. This allowed them to estimate the original impactor size, eccentricity, and volume for Tracks 152 and 163, for densities of 1.9 g/cc and 1.1 g/cc, respectively. These track data offer the ability to search track geometry and chemical maps to identify specific particles of interest for detailed chemical and isotopic analysis. These data are available through NASA JSC Curation and will enable future discoveries of interesting grains and physical properties of impactors in the Stardust sample suite.

**5 Conclusions**

Here we summarize the most important Wild 2 mineralogical and compositional results which provide constraints to early solar system evolution. As expected, the cometary solids were mineralogically similar to anhydrous chondritic IDPs with the exception that GEMS have not between reliably reported in the former. Important similarities include the very wide range of olivine and pyroxene bulk compositions (including minor elements), which probably reflects a wide range of sampled grain formation environments. The abundance of presolar materials in the Wild 2 samples is also approximately equal to that in anhydrous chondritic IDPs rather than being dramatically higher. High-temperature aggregates including chondrules (mainly type II), CAI and possibly AOAs are also present in Wild 2, but probably at less than 10% overall total abundance. There was a pre-mission expectation that comet solids would be dominated by either non-crystalline materials, nanoscale crystalline materials, or presolar grains. While it initially appeared that none of these expectations proved to be correct, it has become apparent that the preferential destruction of the finest-grained Wild 2 material during capture in the aerogel has largely erased its presence. Only further work on the rare occurrences of surviving fine-grained Wild 2 material will reveal its true nature. Organic matter was not very well preserved in the comet Wild2 samples because of the collection process. However, organic phases were preserved in some samples. These materials showing complex organic bonds, including mainly alkenes, aromatic C = C and carboxyl C = O, in a variety of textures, including organic nanoglobules. The organic matter in Wild 2 samples show spectral similarities with that of IOM from carbonaceous chondrites.

Chronology of the collected solids has not been well established, but there is no evidence
for captured live $^{26}$Al, which could indicate comet accretion after this nuclide had effectively
completely decayed. The implications of these major results are that Wild 2 sampled widely
across the entire solar nebula, much wider than any other known astromaterials, potentially
at a relatively late time compared to the oldest available astromaterials. However, a final
caution is that the captured Wild 2 materials described here derived from a small number
(possibly as low as 3) interior regions of a single Jupiter Family comet.
There is also limited mineralogical information from comet 67P provided by the Rosetta Mission. The investigated 67P dust particles also contained about 45 weight% of solid organic matter, bearing similarities with the IOM extracted from carbonaceous chondrites. The H/C ratio of 67P IOM is about twice that of most carbonaceous chondrite IOMs and about 5 times that measured in asteroid Ryugu whole-rock samples, but lower than the H/C ratio of volatile species measured in the 67P coma. This high H/C ratio in the coma species is in turn compatible with that of the soluble organic matter in investigated carbonaceous chondrites such as Murchison. The average N/C ratio of 67P dust particles is about one order of magnitude lower than the photosphere value, but it is compatible with that of primitive carbonaceous chondrite meteorites and of Ryugu IOM samples.

Only future work can establish how these two comets compare to the bulk of comets. Unfortunately, there are no additional comet sample analysis or return missions definitely planned at this time.

There is now sufficient data on the samples returned to Earth by the Hayabusa2 and OSIRIS-REx missions to C-complex asteroids Ryugu and Bennu, respectively, to permit comparisons to be made with comets Wild 2 and 67P. Ryugu and Bennu are both 2nd generation (at least) asteroids, formed from reaccretion of a small fraction of the fragments from much larger disaggregated earlier-generation bodies (Nakamura et al. 2022; Nakashima et al. 2023; King et al. 2024). While the precursor material for the Ryugu and Bennu asteroids may have been mineralogically similar to Wild 2 and 67P, both asteroids have experienced thorough, in fact almost complete, alteration by the activity of a brine – aqueous alteration, though not at a sufficiently high temperature for it to have been actual hydrothermal alteration. Amorphous and crystalline silicates, metal, oxides, sulfides, phosphides, etc., have been dissolved and subsequently replaced by an assemblage characteristic of the CI chondrites – namely saponite, serpentine, Fe-Ni sulfides, magnetite, Ca-Mg-Fe carbonates, Ca-Mg-Na phosphates and evolved organics, to name the most abundant phases. Only minor amounts of the pre-aqueous alteration mineralogical assemblage has survived in the sampled asteroids. Hydrous chondritic IDPs have suffered the same fate (Rietmeijer 1998). Wild 2 and, apparently, 67P escaped this processing, having as final bodies never having witnessed internal temperatures or pressures sufficiently high to melt water ice. It is interesting to note that the two sampled C-complex asteroids are effectively identical to a very rare class of meteorites (CI). We have no way, currently, of understanding how representative 81P/Wild 2 and 67P are of Jupiter Family comets.

**Acknowledgements** This paper benefitted greatly from detailed reviews by Don Brownlee and one anonymous individual.

**Funding** MZ was supported by the NASA Stardust and Hayabusa2 Missions Participating Scientist Programs.

**Declarations**

**Competing Interests** The authors declare that they have no conflict of interest.

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## Authors and Affiliations

Michael Zolensky¹ · Cecile Engrand² · Tomoki Nakamura³ · Denton Ebel⁴

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M. Zolensky

michael.e.zolensky@nasa.gov

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Springer
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T. Nakamura

1 ARES, NASA Johnson Space Center, Houston TX, USA

2 CNRS-University Paris Sud, 91405 Orsay Campus, Paris, France

3 Department of Earth Science, Tohoku University, Sendai, Japan

4 Division of Mineralogy, American Museum of Natural History, New York NY, USA