Presolar Stardust in Asteroid Ryugu

Ancient stardust grains occur as trace components in primitive extraterrestrial materials. These tiny (mostly submicron) grains condensed in the outflows or explosions of evolved stars, prior to the formation of the Sun (i.e., "presolar"). They can be distinguished from other solar system materials by their highly anomalous isotopic compositions that reflect nucleosynthetic processes in their parent stars. Studying presolar grains in the laboratory allows unique insights into galactic, stellar, interstellar, and asteroidal evolutionary processes (e.g., Zinner 2014; Nittler & Ciesla 2016). With nanoscale secondary ion mass spectrometry (NanoSIMS), the compositions and abundances of presolar grains in astromaterials can be determined in situ. This is particularly useful to compare the characteristics of presolar dust among various extraterrestrial samples, such as chondritic meteorites, interplanetary dust particles, and samples returned and to be returned by spacecraft from comets (e.g., NASA Stardust mission) or asteroids (JAXA Hayabusa, Hayabusa2, and NASA OSIRIS-REx missions).

Most presolar grains are either O-rich (oxides, silicates) or C-rich phases (e.g., SiC, graphite), although rare nitrides have also been identified (Nittler et al. 1995). Silicates, the most common presolar phase, can only be studied in relatively pristine samples as they are easily destroyed by secondary processes on asteroids or on Earth (Floss & Haenecour 2016; Nittler et al. 2021; Barosch et al. 2022a). SiC and oxide grains are much more resilient to aqueous alteration and were even detected in highly altered meteorites such as the Ivuna-type (CI) chondrite Orgueil (Hutcheon et al. 1994; Huss & Lewis 1995). In Orgueil, a presolar SiC abundance of $14\mbox{--}{29}_{-6}^{+6}$ ppm was estimated from noble gas analyses (Huss & Lewis 1995) and ${34}_{-8}^{+11}$ ppm was found by NanoSIMS measurements of acid-resistant organic-rich residues (Davidson et al. 2014). Most presolar oxides in Orgueil were found in acid-resistant residues and abundances are not well quantified (Hutcheon et al. 1994; Dauphas et al. 2010; Qin et al. 2011; Nittler et al. 2018a; Liu et al. 2022). No in situ study of presolar grain abundances in CI chondrites has been reported to date.

Based on their isotopic compositions, presolar grains can be classified into different groups that link them to their likely stellar sources. The origins of presolar grains and the interpretation of their isotopic compositions have been extensively discussed in the literature (Zinner 2014; Floss & Haenecour 2016, and references therein; Hoppe et al. 2021). Most grains with O-anomalous isotopic compositions belong to one of four O-isotopic groups and are believed to have formed in the winds of asymptotic giant branch (AGB) stars with different masses and/or metallicities (the majority of Group 1–3 grains), or in supernovae (Group 4 and some Group 1 grains). The classification of SiC grains is based on their C, N, and Si isotopic compositions: They are divided into mainstream, AB, C, N, X, Y, and Z grains and linked to AGB stars (mainstream, Y, Z), ejecta of novae (N), supernovae (X, C), or of ambiguous origin (AB). Presolar graphite grains are mainly thought to come from AGB stars and supernovae.

The Hayabusa2 spacecraft recently collected and returned ∼5.4 g of material from the near-Earth asteroid (162173) Ryugu (Watanabe et al. 2017; Morata et al. 2020; Tachibana et al. 2022). Ryugu is a C-type (carbonaceous) asteroid with a mineralogical, bulk chemical, and isotopic composition that closely resembles CI chondrites (Ito et al. 2022; Nakamura et al. 2022; Yada et al. 2022; Yokoyama et al. 2022). The samples consist of carbonate, magnetite, and sulfide grains embedded within a phyllosilicate-rich matrix. This mineralogy indicates that extensive aqueous alteration has occurred during water–rock interaction on the Ryugu parent body. CI chondrites also experienced extensive aqueous alteration on their parent bodies but they might have been further modified on Earth (Brearley 2014). Thus, the returned Ryugu samples provide a unique opportunity to characterize and ascertain their inventory of preserved presolar grains, to compare their abundances and characteristics to the most similar chondritic samples (CI), and to learn about their origins and history. Furthermore, presolar grain abundances may help identify small-scale heterogeneities in the Ryugu samples, such as in the degree to which Ryugu was modified on its parent body, or heterogeneities in the distribution of presolar material within and between samples.

We analyzed several samples collected by the Hayabusa2 spacecraft during its two touchdowns on the asteroid Ryugu (sample chambers A and C, respectively), as well as material from CI chondrites Orgueil and Ivuna. The following sample types were investigated: (i) polished thin sections A0058-C1002 (from chamber A; Figure 1(a)), C0002-C1001 (from chamber C; Figure 1(b)), and Ivuna HK3 (abbreviated as "A0058-2," "C0002," and "Ivuna" in the following; see Yokoyama et al. 2022 for preparation details). (ii) Small Ryugu (A0108-13, C0109-2) and Orgueil grains. These <1 mm-sized grains were crushed between glass slides. A few dozen ∼10–30 μm-sized particles were then extracted with a micromanipulator and pressed into annealed gold foil with quartz windows (Figure 1(c)). Grains A0108-11 and C0109-8 were sectioned with an ultramicrotome into several ∼250 nm thick slices and placed onto Si wafers (see Yabuta et al. 2022 for preparation details). (iii) Lastly, we received insoluble carbonaceous residues that were prepared by acid treatment of Ryugu grains A0106 and C0107 by Yabuta et al. (2022). The residues were deposited onto diamond windows. All samples were Au coated.

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The thin sections were documented with a scanning electron microscope (SEM; JEOL 6500F) and several fine-grained areas were selected for NanoSIMS analyses (see Figure 1(a)). These were systematically searched for O- and C-anomalous grains by rastering contiguous 10 × 10 μm-sized areas with the CAMECA NanoSIMS 50L ion microprobe at the Carnegie Institution, using a Cs⁺ primary beam (∼100–120 nm, ∼0.5 pA). Ion images of ¹²C⁻, ¹³C⁻, ¹⁶O⁻, ¹⁷O⁻, ¹⁸O⁻, ²⁸Si⁻, ²⁷Al¹⁶O⁻, and secondary electrons were recorded in multicollection mode. Each area was analyzed for 25 sequential cycles with a resolution of 256 × 256 pixels and a 1500 μs counting time per pixel per cycle. The pressed particles and organic residues were primarily analyzed to characterize the microscale isotopic variations of organic matter (Barosch et al. 2022b; Yabuta et al. 2022) and were thus not analyzed for O isotopes. Instead, we used the same primary beam conditions to measure ¹²C₂ ⁻, ¹²C¹³C⁻, ¹²C¹⁴N⁻, ¹²C¹⁵N⁻, plus ¹⁶O⁻, ²⁸Si⁻, and ³²S⁻ or ²⁴Mg¹⁶O⁻, and secondary electrons. An electron gun was used for some measurements to compensate for sample charging. For pressed particles and organic residues larger than 25 μm, the resolution of the ion maps was increased to 512 × 512 pixels. The counting time was 1000 μs per pixel per cycle and 40 sequential cycles were recorded. To better distinguish C-anomalous grains from surrounding materials, some of them were remeasured for N and ²⁸Si⁻, ²⁹Si⁻, and ³⁰Si⁻ isotopes with a higher pixel resolution than used during the initial mapping.

We used the L'image software for data reduction following the protocol described by Nittler et al. (2018b). The ion images were corrected for a 44 ns dead time, for shifts between image frames, and for effects of quasi-simultaneous arrival (Slodzian et al. 2004; Ogliore et al. 2021). In each map, O, C, and Si isotopic ratios were internally normalized to the average composition of each image. N isotopic ratios were normalized to atmospheric N and corrected for instrumental mass fractionation using synthetic SiC–Si₃N₄ (assumed to have atmospheric ¹⁵N/¹⁴N ratios). Presolar grain candidate regions of interest (ROIs) were identified in sigma images of δ¹⁷O, δ¹⁸O, and δ¹³C, in which every pixel represents the number of standard deviations from the average values (see Table 1 for the δ-value calculation). The errors were determined from Poisson statistics (see Nittler et al. 2018b), which completely dominate the measurements. ROIs include all contiguous pixels within the FWHM of the anomalous region(s) and were considered presolar grains if their isotopic compositions significantly differed from the average compositions. Based on image simulations by Barosch et al. (2022a), we chose the following significance thresholds: 5σ for O- and C-anomalous grains with diameters <200 nm, 4σ for grains with diameters >200 nm, and 3.5σ for C-anomalous grains that were clearly associated with ²⁸Si in the ion images (Figures 1(d), (e)). A 120 nm beam broadening correction was applied to grains with sizes below 250 nm (Barosch et al. 2022a).

In the following, we classify C-anomalous presolar grains as SiC grains if ²⁸Si was detected in the ion image or carbonaceous grains if no ²⁸Si was detected. Carbonaceous grains could be presolar graphite, organic matter, or tiny and extremely ¹³C-rich SiC grains for which isotopic dilution has erased the intrinsic Si signal (Nguyen et al. 2007). We attempted to determine the mineralogical compositions of O-anomalous grains with the SEM using an Oxford Instruments energy-dispersive X-ray spectrometer (EDX; 5–10 kV accelerating voltage, 1 nA beam current).

3.1. Presolar Grain Abundances in Ryugu and CI chondrites

We detected a total of 3 O-anomalous presolar grains, 38 SiC grains, and 16 C-anomalous carbonaceous grains among all of the Ryugu samples. The search in the Orgueil and Ivuna samples resulted in the identification of 15 SiC grains and 6 carbonaceous grains. No O-anomalous grains were found in Ivuna, and no O isotopes were measured in Orgueil. All identified presolar grains are listed in Table 1.

Total areas of ∼38,700 μm², ∼25,300 μm², and ∼46,500 μm² were analyzed in Ryugu chamber A samples, chamber C samples, and CI chondrites, respectively. However, presolar grain abundances were only determined from the thin-section and pressed-particle data. To calculate matrix-normalized presolar grain abundances (Table 2), the total area covered by presolar grains was divided by the total area analyzed, excluding large fragments, cracks, or holes in the samples. Ryugu chamber A samples contain ${1.2}_{-1.0}^{+2.8}$ ppm O-anomalous grains (1σ uncertainties based on Poisson statistics and the uncertainty limits of Gehrels 1986), ${24}_{-7}^{+9}$ ppm SiC, and ${17}_{-5}^{+8}$ ppm carbonaceous grains. Chamber C samples contain ${10}_{-7}^{+13}$ ppm, ${26}_{-8}^{+11}$ ppm, and ${4.2}_{-2.7}^{+5.5}$ ppm, respectively (these abundances include both clasts mentioned below; Figure 2(a)). The CI chondrite samples contain ${23}_{-6}^{+7}$ ppm SiC and ${9.0}_{-3.6}^{+5.4}$ ppm carbonaceous grains.

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Table 2. Matrix-normalized Presolar Grain Abundances (ppm) in Ryugu Samples and CI Chondrites

				O-anomalous Grains			SiC Grains			Carbonaceous Grains
Sample	Sample Type	Chamber	Measured Area (μm2)	#grains	abun. (ppm)	error +/−	#grains	abun. (ppm)	error +/−	#grains	abun. (ppm)	error +/−
A0058-2	thin section	A	18408	1	1.2	2.8/1.0	9	25.3	11.6/8.2	9	20.7	9.5/6.8
A0108-13	pressed particles	A	4631	not measured			4	17.8	14.0/8.5	0	0.0	⋯
C0002 matrix	thin section	C	3674	0	0.0	⋯	0	0.0	⋯	0	0.0	⋯
C0002 clast 1 a	thin section	C	4907	2	25.4	33.5/16.4	4	47.4	37.5/22.7	1	7.0	16.2/5.8
C0002 clast 2 a	thin section	C	3695	0	0.0	⋯	3	11.5	11.2/6.2	1	8.9	20.5/7.4
C0109-2	pressed particles	C	3900	not measured			3	37.3	36.3/20.3	0	0.0	⋯
Ivuna	thin section		23110	0	0.0	⋯	12	23.6	9.0/6.7	5	8.1	5.5/3.5
Orgueil	pressed particles		8054	not measured			3	19.7	19.2/10.7	1	11.5	26.5/9.5
(combined) b	Ryugu A		23039	1	1.2	2.8/1.0	13	23.8	8.6/6.5	9	16.6	7.6/5.4
	Ryugu C		16176	2	10.1	13.4/6.6	10	26.0	11.1/8.1	2	4.2	5.5/2.7
	Ryugu total		39215	3	4.8	4.7/2.6	23	24.7	6.3/5.1	11	11.4	4.6/3.4
	CI chondrites		31164	0	0.0	⋯	15	22.6	7.5/5.8	6	9.0	5.4/3.6

Notes. Errors are 1σ (Gehrels 1986). Abun. = abundances.

^a Clast 1+2 are less altered lithologies in section C0002 (Figure 1(b); Kawasaki et al. 2022). ^b Total area measured for C-anomalous grains in Ryugu; areas analyzed for O-anomalous grains are slightly smaller as no O isotopes were measured in the pressed particles.

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All NanoSIMS maps in Ryugu chamber A and the CI chondrite samples were placed randomly in the fine-grained matrix. In section C0002, about one-third of the NanoSIMS maps were placed in the fine-grained matrix (devoid of presolar grains) and the other two-thirds were placed in two areas with less altered lithologies detected by Kawasaki et al. (2022) (clasts 1+2 in Table 2; Figure 1(b); see Section 4). Both O-anomalous grains in section C0002 were found in clast 1. The presolar grain abundances for clast 1 are ${25}_{-16}^{+33}$ ppm O-anomalous grains, ${47}_{-23}^{+38}$ ppm SiC, and ${7.0}_{-5.8}^{+16.2}$ ppm carbonaceous grains. Clast 2 contains no O-anomalous grains, ${11}_{-6}^{+11}$ ppm SiC, and ${8.9}_{-7.4}^{+20.5}$ ppm carbonaceous grains.

3.2. Presolar Grain Compositions

Two O-anomalous grains (HY2-O-01 and -02; Table 1a) are characterized by enrichments of ¹⁷O and subsolar to solar ¹⁸O/¹⁶O ratios (Group 1; Figure 3(a)), which are believed to mostly indicate formation in low-mass AGB stars (<3 M_⊙; Zinner 2014). Grain HY2-O-03 is an ¹⁸O-enriched Group 4 grain (likely supernova origin) with a ¹⁷O/¹⁶O ratio close to solar. Grain HY2-O-01 was identified among fine-grained matrix in section A0058-2 and is probably an oxide (Figure 1 (f)). It is associated with Al in the ion image, which is suggestive of an oxide grain and silicates are unlikely to survive the extensive aqueous alteration seen in most of the Ryugu samples (Yokoyama et al. 2022). Grains HY2-O-02 and -03 are both from the same less altered lithology in clast 1 of section C00024. Based on the presence of Si in the EDX map and the absence of Al in the EDX map and the ion image, HY2-O-02 appears to be a silicate (Figures 1(g), (h)). Grain HY2-O-03 has the same Si⁻/O⁻ and AlO⁻/O⁻ ratios as the surrounding material and could also be a silicate, although the grain is very small (∼120 nm) and it was not possible to cleanly measure its composition.

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The C and N isotopic compositions of presolar C-anomalous grains are compared to literature data in Figure 3(b) and Si isotopic compositions are displayed in Figure 3(c). Grains without N and Si isotopic data cannot be classified reliably (Tables 1b, c). Most Ryugu SiC grains are plotted in the region of mainstream grains in Figure 3(b) and are linked to AGB stars (Zinner 2014). At least two grains with ¹²C/¹³C ratios of ∼5 and ∼8 are most likely AB grains, and one Z grain (HY2-C-05) was identified by its location on the Si three-isotope plot (Figure 3(c)). In the CI chondrite samples, most SiC grains are probably mainstream grains with ¹²C/¹³C ranging from 12 to 59. Two AB grains with ¹²C/¹³C ≈8 were identified (Ivuna-C-10 and -16). SiC grain diameters range from <100–450 nm, with an average diameter of ∼240 nm.

Moderate C-anomalous isotopic compositions of approximately δ¹³C ≈300‰ to −300‰ (¹²C/¹³C ≈68–127), and a large diversity in δ¹⁵N are typical for ∼1% of the C-rich organic grains that are present ubiquitously in Ryugu (Barosch et al. 2022b; Yabuta et al. 2022). Most carbonaceous grains with similar compositions are probably organics. However, the majority of carbonaceous grains reported here have much more anomalous compositions, i.e., with the lowest ¹²C/¹³C value at 13 and the highest at 224. Some of these could be SiC grains but at least two grains (HY2-C-27, -48) with very low Si⁻/C⁻ ratios are most likely presolar graphites. The C-anomalous grain sizes range from 110–750 nm with an average size of ∼270 nm.

Two presolar SiC grains in Ryugu samples were recently detected by Yabuta et al. (2022) and a presolar graphite grain was detected by Ito et al. (2022). Here, we significantly expand on this work. All major types of presolar grains typically found in situ in carbonaceous chondrites were identified in Ryugu: oxides, silicates, SiC, graphites, and C-anomalous organics (Table 1). The detection of at least one presolar silicate (Figures 1(g) and (h)) in Ryugu was particularly unexpected, as silicates are easily destroyed during aqueous alteration (Floss & Haenecour 2016). Their occurrence is probably restricted to relatively rare clasts with less altered lithologies than the typical Ryugu matrix, as described by Kawasaki et al. (2022) (clast 1 in Figure 1(b)). These contain anhydrous minerals such as Mg-rich olivines and low-Ca pyroxenes, which indicate that these clasts escaped extensive alteration. The inferred abundance of O-anomalous grains in C0002 clast 1 is ${25}_{-16}^{+33}$ ppm (2 grains; 4907 μm² analyzed; Figure 2(a); no O-anomalous grains were found in clast 2) and consistent with fairly altered CM2 and CR2 chondrites (Leitner et al. 2020, and references therein). Nguyen et al. (2022) reported an abundance of ${165}_{-90}^{+161}$ ppm (three O-anomalous grains; 1072 μm² analyzed) for a similar clast in another Ryugu chamber C section. While much more analysis is required to determine reliable abundances, it is clear that these clasts were able to preserve delicate presolar grains, whereas the typical Ryugu matrix was not.

No O-anomalous grains were detected in situ in Ivuna, although oxides are known to exist in CI chondrites (Hutcheon et al. 1994). Assuming an average presolar grain size of 0.25 μm, we estimate a 1σ upper limit for the nondetection for the abundances of O-anomalous grains of ∼4 ppm (Gehrels 1986). This upper limit is fully consistent with the abundances seen in the Ryugu matrix (${1.2}_{-1.0}^{+2.8}$ ppm) and the initial estimate of ∼0.5 ppm by Hutcheon et al. (1994) based on a single large presolar Al₂O₃ grain from Orgueil. Recent studies by Morin et al. (2022) and Kawasaki et al. (2022) report Ivuna clasts that contain anhydrous primary minerals, which are likely derived from amoeboid olivine aggregates and/or chondrules. These lithologies are similar to the less altered lithologies in Ryugu and might be interesting future targets for presolar grain analyses.

The inferred abundances of SiC grains are consistent across the Ryugu chamber A and C samples (${24}_{-7}^{+9}$ ppm versus ${26}_{-8}^{+11}$ ppm) and essentially the same abundances seen in the CI chondrite samples (${23}_{-6}^{+7}$ ppm). Independent (ex-situ) estimates of the SiC abundance in CI chondrites are similar within uncertainties (Figures 2(a) and (b); Huss & Lewis 1995; Davidson et al. 2014). This result is thus consistent with the Ryugu–CI chondrite connection (Ito et al. 2022; Nakamura et al. 2022; Yada et al. 2022; Yokoyama et al. 2022); however, comparable SiC abundances are also found in other unheated carbonaceous chondrites (Figure 2(b)).

SiC grains were relatively homogeneously distributed across the Ryugu thin sections, with at least one SiC found in most Ryugu matrix regions measured (Figure 1(a)). In contrast, their distribution in Ivuna seems much more heterogeneous: 60% of all grains were detected in relatively close proximity in a ∼0.5 × 0.5 mm-sized matrix region which was indistinguishable by SEM from the other regions measured, with a factor of 3 lower number density in the other regions. The abundances of carbonaceous grains seem to be slightly higher in Ryugu chamber A samples compared to chamber C samples (${17}_{-5}^{+8}$ ppm versus ${4.2}_{-2.7}^{+5.5}$ ppm). These differences might indicate some heterogeneity in the distribution of presolar material in the matrices of these samples.

As in most other in situ studies, the presolar grain abundances reported here are lower limits. In particular, small grains (<100 nm) can only be reliably identified by higher-resolution NanoSIMS analyses (Hoppe et al. 2015; Nittler et al. 2018a). Some presolar grain types found in Orgueil, such as nanodiamonds (Zinner 2014) and presolar oxide grains with anomalies in isotopes other than oxygen (e.g., ⁵⁴Cr and ⁵⁰Ti; Dauphas et al. 2010; Qin et al. 2011; Nittler et al. 2018a ), were missed in this study.

The presolar grains detected in Ryugu have isotopic compositions consistent with those seen in primitive meteorites (Zinner 2014, and references therein). Compared to the isotopic compositions of SiC grains detected in acid residues (see presolar grain database; Stephan et al. 2020), the N isotopic ratios for many Ryugu SiC grains are closer to solar and/or terrestrial (Figure 3(b)). Moreover, the ¹²C/¹³C ratios of five Ryugu SiC grains are between 10 and 20 (Table 1b), which is an unusual value, falling between the AB and mainstream SiC populations. Both observations can be explained by the ubiquity of organic matter in Ryugu (Barosch et al. 2022b; Yabuta et al. 2022). Indeed, it was not always possible to completely disentangle the C and N signals arising from organics from those intrinsic to the presolar grains. The contribution of C and N from surrounding organic matter to the grains dilutes the isotopic signatures of presolar SiC grains and shifts them toward less anomalous compositions, as shown by the green model mixing curves in Figure 3(b). These curves indicate mixing between six select "true" SiC compositions and bulk organic matter in Ryugu (¹²C/¹³C ≈ 90, ¹⁴N/¹⁵N ≈ 260; Yabuta et al. 2022) and show how C and N contamination from the organic matter leads to a narrower range of SiC compositions compared to literature data. This could also lead to misclassification and/or nonidentification of C-anomalous presolar grains with low to moderate anomalies in Ryugu and CI chondrites.

The samples returned from asteroid Ryugu by the Hayabusa2 spacecraft contain presolar stardust grains. Their abundances and compositions are similar to presolar material found in CI chondrites. Thus, our results provide further evidence that asteroid Ryugu is closely related to CI chondrites, a connection originally based on mineralogical and bulk chemical and isotopic data (Ito et al. 2022; Nakamura et al. 2022; Yada et al. 2022; Yokoyama et al. 2022).

Refractory O- and C-rich presolar phases survived the pervasive aqueous alteration that Ryugu has experienced, whereas delicate presolar silicates were likely destroyed. However, small regions of Ryugu escaped extensive alteration (Kawasaki et al. 2022; Nakamura et al. 2022) and allowed their preservation. Further analyses of less altered Ryugu lithologies would be highly beneficial to better characterize their inventory of preserved presolar material and compare it to more altered Ryugu matrix.

Petrographically and isotopically similar less altered clasts were recently detected in Ivuna (Kawasaki et al. 2022; Morin et al. 2022) and could be targeted in future studies for comparison with the Ryugu samples. These clasts might contain presolar silicates that have not yet been found in Ivuna. The presence or absence of presolar material in these clasts would provide important clues about their origin and their history of secondary processing.

Future NanoSIMS-based analyses of Ryugu samples will focus on particles with shallower 2.7 μm OH absorption features in their infrared reflectance spectra (see, Yada et al. 2022). These may be less aqueously altered. Their study will allow us to better assess the scale of heterogeneity sampled on Ryugu, and to explore the effects of differing degrees of alteration on organics (see Yabuta et al. 2022) and presolar grains. Systematic searches for presolar grains in all Ryugu lithologies will provide a representative data set of presolar grain abundances and characteristics in asteroid Ryugu and will extract the maximum scientific information from these precious samples.

This work was carried out under the auspices of the Hayabusa2 Initial Analysis Team, specifically the Chemistry subteam led by Prof. H. Yurimoto and the Macromolecule subteam led by Prof. H. Yabuta. We thank Nico Küter for assistance during sample preparation. This work was funded in part by NASA grants NNX16AK72G and 80NSSC20K0340 to L.R.N.